My Vehicle Products Diagnostics Videos Photos
2001-2007 6.6 L Duramax Diagnostics

High Pressure Common Rail Basic Information
 The high pressure pump builds rail pressure and delivers it to the fuel rail manifold where it flows through the injector lines to the injectors. The fuel pressure regulator in the high pressure pump controls rail pressure. The injectors have a hollow check ball that holds high pressure fuel until the fuel solenoid is actuated by the ECM, this allows the check ball to rise off its’ seat and an injection to take place. If the check ball in the injector is leaking due to erosion on the seat or the high pressure limit valve leaks then it will not build enough rail pressure to start. It takes approximately 2500 PSI 2500 PSI rail pressure to start.

common rail injector cutaway

 The fuel system contains high pressure fuel up to 25,000 PSI. Do Not use you fingers to find fuel leaks! High pressure fuel entering your bloodstream may result in amputation or loss of life.

Preliminary checks

  1. Check engine oil level, LB7 engines have the injectors under the valve cover and are very susceptible to dilution.
  2. Check and record any DTC, look at snap shot data or save, do not erase codes prior to doing repairs, you will erase the snapshot data.

No Start or Hard Start

  • Excessive fuel restriction, check or change fuel filter
  • Use vacuum gauge, check the suction, you should have no more than 5 inches Hg at WOT (wide open throttle) or 7-8 inches Hg under load. If you have too much suction the restriction could also be the sock in the tank. Too little vacuum (less than 2 inches Hg) means that it could be sucking air.
  • Check for air in fuel system, install clears lines to check.
  • Confirm actual versus desired rail pressure, even under crank no start conditions
  • If the above are ok, then it comes down the following.
    • fuel injectors (see injectors for more diagnostic information)
    • high pressure injection pump
    • Fuel pressure regulator, check to make sure it is not stuck.
    • fuel pressure relief valve (limit valve), check to make sure it is not leaking into the return system when rail pressure is 160 Mpa
  • Before condemning the high pressure injection pump you need to make sure there are no high pressure fuel leaks. Use the EN-47589 (same block-off caps as #9011 SPX Miller tools) cap set to cap off the injector rail to isolate the injectors from the injector return system.

Black Smoke

  1. If at idle, use the scan tool to cut out one cylinder at a time and see if the smoke disappears.
  2. Dirty air filter
  3. Exhaust leaks or Boost leaks, you can usually hear a boost leak as a high pitched squeal under load.
  4. EGR and or MAF problems or intake leaks after the MAF sensor.


  1. Use scan tool to isolate one cylinder at a time. Run the injector balance test, running the test hot (after a hard drive) and in drive will give the most consistent results.
  2. A missing or damaged chamber gasket or low compression could all cause a miss.


  1. Use scan tool to isolate one cylinder at a time.
  2. Use cap off tool EN-47589 to block off one injector at a time.

Surge at idle or "lopey idle", lopes at idle

  1. Map actual versus desired rail pressure, if the graph is wavy and there is no air in the system it is usually caused by a bad fuel pressure regulator.
  2. Air in the fuel system (see fuel supply and filter housing section)

White-Blue smoke at idle when cold
 If the smoke clears in less than 1 minute, this would be normal depending on temperature and altitude. Blue white smoke that burns your eyes is un-burnt fuel; cold temperatures, high altitude and excessive idle time all mean cold combustion and white smoke.

  1. Possible bad injector, use the scan tool to cancel one cylinder at a time and see if the smoke clears up. However, using the scan tool to kill the injector does not reduce rail pressure in the injector and the tip can still leak fuel, cap off lines one at a time (cap is tool # EN-47589) to pinpoint injector. Also look at the balance rates, if the tip is leaking fuel then the balance rates may be out of specification.
  2. Check glow plug operation when cold.
  3. Check rail pressure when engine is off, it should 1.0-1.8 Mpa, depending on the engine model.
  4. Excessive idle time can cause white smoke when cold due to carbon build up on injector tips. More than 20% idle time is excessive. If the injectors have excessive carbon on the nozzle tip then balance rates should be high on that cylinder.


  1. Cracked injector
  2. Injector return lines are under the valve cover on the LB7. Pressurize the return circuit with the valve covers off and look for bubbles or vacuum test the return circuit off of each head, the return should hold 15 inches Hg of vacuum.
  3. On the LB7 dilution problems that occur after you have replaced the injectors could be return line leaks under the valve cover or leaks at the injector line to injector connection. The green coated injector lines are more susceptible to leaks versus the polished steel colored lines.
  4. Leak at the high pressure pump driveshaft seal.
  5. You can also use a dye in the fuel and black light kit to try to pinpoint leaks at the injectors on an LB7. Follow the “fuel leaks inside of engine” GM service information.

Fuel Supply and Fuel Filter Housing
 The fuel filter housing is on the suction side (there is not a supply pump from the factory) and are prone to suck air. Follow the GM fuel system diagnosis in the service manual.

  1. Install fuel vacuum test tool.
  2. Prime the fuel system with the hand primer until 10 PSI is indicated on the gauge, check for external leaks and repair. If the pressure drops from 10 PSI to 2 PSI in less than 1 minute, remove the fuel outlet line from the filter and cap it. Remove the ignition 1 relay and crank the engine for 2 - 15 second intervals, the high pressure pump should pull at least 12 inches of Hg vacuum.
  3. Install clear hoses at the inlet and outlet of the fuel filter housing. Re-prime the system and then start the engine, there should be very little air going into or coming out of the fuel filter housing.
  4. Common air ingestion places are the filter housing, plugged filter, drain valve, rubber hoses and connections. You need to use clear lines to isolate where the air is coming from and work your way back toward the tank until you don’t have any more air coming through the clear line. Unless you know where to get the tool that sees through back rubber lines to find air, your only other option is to bounce around and replace parts.

High Pressure Pump (CP3 Pump)

  1. Before condemning the pump for a starting issue you need to make sure that the high pressure fuel system is not leaking the pressure.
  2. If there has been a major contamination issue with dirt and or water then it is very likely that the high pressure pump will need to be replaced. However, the injectors are typically damaged first, but any contamination that got into the injectors also went through the CP3 pump.
  3. The most common failure of the high pressure pump is the inability to keep up with high fuel demand such as towing a trailer up a hill. This problem will usually set a low rail pressure code.

 It takes about 2500 PSI rail pressure for the injectors to deliver fuel and the engine to start.

  1. Injector return flow; maximum allowable leakage for one injector is 5 ml in 15 seconds, maximum per bank is 20 ml: check when cranking, with the FICM disabled, pressure should be 114 – 135 Mpa during cranking. Specifications are for API rating of 40-44.
  2. GM only provides a specification for return flow when cranking however, we have done some testing on a good running LB7 and found that injector return flow at idle from one bank was 95 – 110 ml in one minute at 21,000 PSI.
  3. Excessive leakage from the injectors usually results in a starting issue, which could occur hot or cold, but usually occurs hot because the fuel is thinner when hot. However, excessive leakage from the injectors can also cause a DTC P0087, P0093 or a P1093 to set. When using the scan tool to increase rail pressure at idle, if you can’t get to 21,000 PSI then the injectors are usually bad.
  4. You can also use balance rates to help determine if you have any bad injectors. If an injector is leaking excessively into the return the balance rates are often at the edge of specification. Injectors that have a poor cylinder power contribution or a noise or smoke change when cancelled will also need to be replaced and are likely to cause low rail pressure during cranking.


  • 2001-2006; turbo bearing failure may be caused by a spun camshaft bearing. Inspect for a spun number 4 camshaft bearing, refer to TSB 03-06-93-001B.
  • 2004.5 and newer turbochargers have a vane position sensor, check actual versus desired.

Use the following information regarding diagnostic trouble codes in addition to the normal diagnostic procedures outlined in the service manual or technical service bulletins.

DTC P0087; fuel rail pressure less than 22.5 MPa at more than 600 RPM, fuel rail pressure too low.   

  • Excessive restriction, fuel supply, plugged filter or sucking air. Install special tool J44638 to check vacuum restriction on fuel supply to the high pressure pump. Maximum restriction at WOT (wide open throttle) is 5 inches HG in park. When driving under hard acceleration maximum would be 7-8 inches Hg. If too high replace the fuel filter and retest.
  • If it is only a couple of inches vacuum, that could indicate that the fuel supply system is sucking air, use clear fuel lines at the filter head to check for air.
  • Excessive restriction could also cause a DTCP1093 to set at the same time.
  • Rail pressure should read 1-1.8 MPa with key on and engine off. If out of range replace the rail pressure sensor.
  • With the engine up to operating temperature, use the scan tool to command rail pressure to 21,000 PSI, if the rail pressure will not achieve 21,000 PSI at idle you most likely have a problem with the injectors, Especially if you are having a hard start, miss, rough run or smoke.
  • Disconnect the fuel rail pressure sensor the fuel pressure should be greater than 175 Mpa as displayed on the scan tool.
  • If these codes set only on hard acceleration or when pulling a hill with a load, check fuel supply issues first. Then see if rail pressure will reach 21,000 PSI at idle, if it does then the low rail pressure under a hard load is usually caused by a bad high pressure pump.

DTC P0093 on 2001, P1093 on later vehicles; Difference between commanded fuel pressure and actual pressure is greater than 20 Mpa.

  • If P0087 is set solve P0087 first
  • Similar diagnostics to DTC P0087

DTC P0201 – P0208 misfire codes

  • Can be caused by the FICM wire harness rubbing on the back of the alternator, alternator bracket or FICM harness retaining bracket.
  • Air in the fuel system can also cause misfire codes.

DTC P2146 or 2149; cylinder bank shut down

  • Can be caused by the FICM wire harness rubbing on the back of the alternator, alternator bracket or FICM harness retaining bracket.
  • LLY engine (2004.5-2005), #2 (P0202) and #7 (P0207) misfire codes set with codes P2146 and P2149. Refer to TSB 05-06-04-047B to repair the injector harness brackets and connectors (part # 98017958) for #2 and #7 before replacing the FICM or the injectors.
1998-2002 Cummins 5.9L 24 Valve Diagnostics

The fuel supply pump pressure is critical for the longevity of the VP44 injection pump. The fuel supply pump should maintain at least 4psi at the inlet to the injection pump on a W.O.T. test drive. Failure to maintain a pressurized flow of un-aerated fuel will cause injection pump damage.
4psi at the inlet to the injection pump with a clean fuel filter allows for a pressure drop across the fuel filter when it is dirty. According to Chrysler, pressure doesn't matter, only volume. The problem with their volume test with an unrestricted flow is their specifications for volume aren't high enough. Some vehicles with the updated in-tank supply pump won't maintain a positive pressure during a test drive. When the supply pump in the injection pump is pulling more fuel than the supply pump in the tank is pushing, then pressure becomes a vacuum because volume isn't high enough. Continued driving, when fuel is under a vacuum, can cause cavitations, which will damage the injection pump.

In order to maintain good fuel flow, especially with performance modifications, you may need to add an additional supply pump.

NOTE: The following information is not a substitute for the proper diagnostic manuals, but to share some of the common problems we have run across.

Performance boxes that tap into, or connect into, vehicle wiring should be removed to make certain that they are not causing any of the following complaints.

"Normal" life seems to be about 150,000 miles. When injectors are going bad, they can cause many different intermittent problems, such as: no start, hard start, black smoke, low power, white smoke and rough running.

1. Low or no fuel supply pressure.
2. Dirty air filter or inlet restriction.
3. Exhaust leak before the turbo.
4. MAP sensor can go bad and not set a code, check reading with scan tool versus actual.
5. Injectors bad

1. Low or no supply pressure.
2. No fuel - sending unit in tank defective.
3. If the supply pump recently failed, it could damage the injection pump and cause a no start.
4. Check for DTC, follow Powertrain Diagnostic Procedures Manual.

1. Check injectors, remove injection lines and crank engine. Do any of the injector feed tubes pump compression back out while cranking? Replace injector, if needed.
2. CMP sensor defective.
3. Injection pump

1. Follow the same guidelines as no start.
2. Fuel leaks at injector feed tubes, internal or external?
3. If supply pump recently failed, it could cause injection pump problems due to cavitation damage.

1. Check for DTC.
2. Check injectors.
3. Leaking fuel injector tubes may also cause a miss. If they leak externally, they are also leaking at the injector connection.

1. No or low fuel supply pump pressure.
2. 98 & early 99 automatic transmission, see TSB on erratic torque converter clutch operation - needs reflash

1. No or low fuel supply pressure.
2. DTC present? Look at solving the codes first.
3. With black smoke and no DTC, possible bad injectors.

In addition to the information in the Diagnostic Procedure Manuals, consider the following checks and information.

DTC 215 - Fuel Injection Pump Control Circuit
1. Swap injection pump relay with horn relay and retest.

DTC 216 - Fuel Injection Pump Timing Failure
1. If supply pump previously failed (within the last 2-3 months) it could cause advance component and housing damage in the injection pump.
2. Low or no fuel supply pump pressure.
3. Improper installation of replacement injection pump. Key not aligned with gear and gear pulled onto shaft with driveshaft nut.
4. Injection pump drive gear installed one tooth off.

DTC230 - Transfer Pump Circuit Out of Range
1. Bad fuel Supply pump will normally cause this code.
2. Could also cause a DTC216.
3. Could set due to excessive cranking, see hard start diagnostics.

DTC336 - Engine Crankshaft Position Sensor (CKP) Signal
1. Indicates no engine speed or position signal to ECM.
2. Can cause other codes, solve DTC336 first.
3. Reads out on scan tool as ECM engine speed.
4. Will cause multiple problems, such as surge, miss erratic run, hard start, no start, stalls, etc.

DTC370 - Fuel Injection Pump Speed / Position Sensor Signal Lost
1. If no other DTC, pump is likely faulty.

DTC602 - ECM Fueling Calibration
1. Could be caused by a performance box.
2. Can also be caused by a DTC336.

DTC1688 - Internal Fuel Injection Pump Failure
1. This code can be caused by a bad crank sensor (DTC336)
2. If no other DTC, most likely injection pump needs to be replaced due to an electronic failure.

DTC1689 - No Communication Between ECM and Injection Pump Module
1. Can be caused due to excessive cranking - See hard start & no start diagnostics.
2. Wires chaffed at pump connection.
3. Performance box problem.
4. Fuel pump relay bad (swap with horn relay and retest).
5. Possible defective pump.

DTC1690 - Fuel Injection Pump CKP Sensor Does Not Agree With ECM CKP
1. Monitor RPM reading of the following: ECM engine speed (crank position sensor See DTC336) vs. engine speed CMP (cam position sensor) vs. injection pump engine RPM (from injection pump).
2. Can be caused by a DTC336 - repair DTC336 FIRST

DTC1691 - Fuel Injection Pump Controller Calibration Error (PSG)
Likely an injection pump problem.

DTC1693 - DTC Detected in PCM
The JTEC controller is reporting that there is a DTC stored in the PCM.

1994-2000 6.5L with DS4 Injection Pump Duramax Dianostics

1. Low or no fuel supply pump pressure
2. Dirty fuel filter
3. Wastegate control solenoid

1. Low or no fuel supply pump pressure

1. Dirty air filter
2. Waste gate control solenoid failure, or low vacuum.
3. Worn injectors (excess carbon on the nozzle tips)
4. Low or no supply pump pressure
5. If this occurred after the plenum chamber (intake) was removed, check the EGR tower gasket, if not seated EGR gases enter the intake unrestricted and in excessive quantity.
6. Restricted exhaust system, if the vehicle has been “babied”, the cat will partially plug. Take the vehicle out for several full throttle accelerations to clear out the exhaust

1. If the complaint is a hard start after a hot soak, not a stalling condition then hard start, check for a minimum cranking speed hot of 200 rpm.

1. Worn injectors
2. If the engine was overheated, check for a head gasket problem, usually on the turbo side (usually #4 & #6).
3. See Misfire Codes further below.

This is often caused by a bad PMD (Pump Mounted Driver or Fuel Control Solenoid Driver); however it is unusual to have multiple failures of the PMD. A bad Fuel control Solenoid (part of the injection pump) can cause the same symptoms and codes. A bad EGR, EGR vent, wastegate control, and transmission solenoid are on the same quad driver circuit and can cause intermittent stalling.

 We use the following guidelines to determine if a PMD may need to be replaced, this is a simple first step and is less in depth than the GM bulletin noted below.
Replace the PMD after repairing any other problems such as no fuel supply pressure if:

  • If complaint of intermittent dies with no codes and you can’t duplicate it, remember that a bad noise suppression harness can cause intermittent stalling.
  • Intermittent dies with no closure errors (closure errors are usually caused by the fuel control solenoid).
  • Intermittent stalling and C-Time varies, as normal, during acceleration and deceleration (if C-Time is “stuck” at 1.95ms or 2.25ms this usually means a bad fuel control solenoid).
  • If you have a no start and C-Time is 1.95ms (94-95) or 0.01ms (96-2000) replace the PMD
  • C-Time is erratic at idle while idle rpm is stable.

If the problem you are experiencing doesn’t correspond with the problems noted above then follow the trouble shooting below to help pinpoint a possible bad PMD. (GM Bulletin #77-63-06A)

1. Check fuel system for air (check at fuel return from pump), supply pump pressure, and fuel filter condition.

2. Check the PCM and injection pump wiring harness for chaffing and/or loose terminals at the PCM connector, the 15 pin connector and the PMD connector. See the WIRE HARNESS CONNECTIONS section for more information.

3. Check all engine and vehicle harness grounds.

4. Check for loss of ignition voltage on terminal "D" at the fuel solenoid driver connection to the injection pump. Refer to ENGINE CONTROLS IN THE 1998 SERVICE MANUAL FOR ALL MODEL YEARS.

5. Verify that the condition is still present:
 A. If the condition is no longer present, the vehicle is repaired
 B. If the condition is still present, and there are active DTC’s set, go to step 6.
C. If the condition is still present, and there are NO active DTC’s set, go to step 7.

6. For active DTC’s 35 or P1216, refer to Engine Controls in the 1998 Service Manual for all model years. If the DTC diagnostic table flow chart directs you to replace the injection pump, replace the PMD (Pump Mounted Driver) which is also called the Fuel Solenoid Driver. Go to step 9.

7. For stall condition, disconnect the Optical Sensor (Encoder Sensor) and operate the engine in back up mode (you will experience an extended crank time during starting, up to 15 seconds). A bad noise suppression harness or no noise suppression harness can also cause a stalling condition. If the condition is still present replace the PMD only. Go to step 9.

8. For a no start condition, check injection pulse width (94-95) or C-Time (96-2000 Fuel Solenoid Closure Time) with a scan tool, while cranking (maintain minimum cranking speed 100 rpm cold and 200 rpm hot). If C-Time is 1.95ms (94-95) or 0.01ms (96-2000) replace the PMD. Go to step 9.

9. Replacing the PMD (Fuel Solenoid Driver):
 The PMD must be mounted to a “heat sink” of some kind. Stanadyne (Injection Pump Manufacture) states that the PMD must be mounted to the injection pump with the heat pad underneath it. In order to mount the PMD to the injection pump you must remove the intake manifold.
 If you don’t want to remove the intake manifold, you could purchase a remote mount kit such as the BD remote mount kit, and an aftermarket PMD. This would allow you to replace the PMD without removing the intake manifold.
 Stanadyne has finally come out with a new PMD it is completely redesigned from the original PMD. Since the wire harness connections are different as well as the calibration resistor, the intake manifold must be removed in order to replace the PMD, because the wire harness between the PMD and the Fuel Control Solenoid will have to be replaced.

Removing the PMD requires a T15 TORX bit. Replace the heat transfer pad and use snap ring pliers to remove the calibration resistor. Install the calibration resistor into the new PMD.
PMD resistor
 If the calibration resistor is damaged, there is an identification number on it (1 through 9) and we have them available.


INSTALL PMD: Align the mounting screws on the PMD with the holes on the injection pump (or on the remote mount PMD heat sink) Torque the mounting screws to 23 LB-IN.



For Other Starting Issues, Check the high resolution signal, cam signal (from the injection pump) and crank signal.

High Res. Signal






Cam signal






Crank signal






Will it start?

Yes, extended crank




Yes, noisy on startup

X = good signal, 0 = no signal

Hertz Readings (HZ) while cranking and at idle








High Res. Signal

597 Hz

683 Hz

768 Hz

853 Hz

2987 Hz

3413 Hz

Cam and Crank Signal

9 Hz

11 Hz

12 Hz

13 Hz

47 Hz

53 Hz

Light duty engines utilize an EGR system to help the meet emissions. The EGR flow calculation on some models uses the MAF sensor. If there are any “air intake leaks” either MAF or EGR codes can set. A missing or improperly sealing EGR tower gasket can cause either or both codes to set.
   Low or no vacuum from the vacuum pump can also cause EGR codes to set. A missing EGR tower gasket can cause a MAF low code to set, along with excessive black smoke, because it is pulling exhaust gases instead of clean air through the air filter.
 Failure to remove the block off plates from the intake manifold gaskets for a light duty engine can cause EGR codes because there will be no exhaust gases getting to the EGR valve.

The fuel supply pump operates during cranking and after the engine oil pressure comes up. The supply pump is driven from a relay on the firewall or in the under hood fuse/relay center. The engine oil pressure sensor also includes a switch to operate the supply pump once oil pressure comes up after starting.
   The supply pressure should be between 5-7 psi at idle. Test at the fuel filter drain at the front of the engine near the coolant crossover. Be aware that this is a pressure test on the dirty side of the fuel filter and if the filter is dirty the actual supply pressure to the injection pump will be lower.
  If the fuel supply pressure is zero, remove the fuel supply pump relay and jump #30 & #80 connector, this will send power to the supply pump with the ignition on. If you have good supply pressure now, then the most likely cause is a bad oil pressure sending switch.
 Low fuel supply pressure can cause DTC 17 (P0370)and 18 (P0251) to set due to aeration in the fuel, after all it is an optical sensor and it can’t see through an air fuel mix.

Misfire codes are handled differently for different years. Some misfire codes go by firing number order, in other words a misfire code for #2 would be #8 cylinder because it is the #2 cylinder in firing order. Check the service manual for the year you are working on.

 Firing order and cylinder arrangement is;
firing order

The injectors on the 6.5 L are usually only good for about 100,000 miles. They get a lot of carbon blown up inside of the nozzle tip. This results in poor atomization and poor spray patterns.

PMD (pump mounted driver)
The pump mounted driver gets 12 volt power when the key is on from a fuse in the
The PCM then sends a signal to the PMD to close the fuel control solenoid. When the fuel control solenoid closes it sends a signal back to the PMD and that is relayed to the PCM. The response time of the closure signal is called C-time.
  C-Time should change when the engine is accelerated or decelerated. A fixed C-Time can indicate a bad Fuel Control Solenoid. If the C-Time is erratic when the RPM is stable then this usually indicates a bad PMD. See Wire Harness Testing for other possible problems.

If you have a no start condition, disconnect the encoder (optical) sensor and crank the engine for 15-20 seconds. If it starts then the problem is with the optical sensor.
The encoder sensor has to be able to see through the fuel inside the pump, if there is any aireation it can cause trouble codes to set. Dirty fuel, SVO, no fuel supply pressure (air in system) or dyed fuel can all cause a problem with the encoder sensor “reading” the windows in the cam disk. There are 8 windows in the low resolution circuit and 512 windows in the high resolution circuit. In on minute at idle the high resolution circuit needs to read about 180,000 windows, in order to not set a code.

The pump to engine timing must be set with a scan tool. You need to do the TDC offset learn procedure in order to set timing.

The turbo on the 6.5 L EFI engine has a wastegate actuator that is controlled by vacuum. The wastegate actuator on the 94-2000 turbo does not have a spring, when no vacuum is applied the wastegate will “flop” back and forth. If the wastegate actuator holds a vacuum and it moves freely back and forth then there is nothing wrong with the wastegate actuator. A turbo boost problem and DTC78 (P0236) is normally caused by low vacuum (need 20 inches HG) or a bad wastegate control solenoid.
 To inspect the turbo, remove the inlet hose from the compressor inlet and make sure the turbo spins freely and that the compressor wheel does not rub on the housing when pushed to the side. Some side play is normal, because the bearings “float” on oil from the engine when the oil pressure is up.
 The turbo pulls crankcase fumes from the engine through the CDR valve. It is normal for the turbo to discharge oil from the compressor housing. You have to determine if you have more oil coming out versus going into the compressor housing. A plugged air filter or bad CDR valve can cause excessive oil to be pulled out of the engine and discharged from the turbo back into the intake. If the silicone hose connecting the compressor discharge is not sealed with a minor amount of silicone sealant than the hose connection will weep oil past the hose.

The wastegate (WG) control solenoid controls the amount of vacuum going to the wastegate actuator on the turbocharger. The WG solenoid is controlled via a pulse width modulated signal from the PCM.

Poor electrical connections or wiring can cause most intermittent problems. Perform a careful check of the suspected circuit for the following:
Inspect and ensure the integrity of all related wiring harness connectors. If the wiring harness connectors are not properly put together or engaged before they are locked together, numerous types of conditions may occur. This may include many intermittent symptoms and DTC codes.
The first step in any type of electrical diagnosis is that a visual and physical inspection be completed of the wiring harness connectors for integrity. Many times, the vehicle may be repaired just by disconnecting and reconnecting the connectors. As with all repairs to wiring harness connectors and terminals, a pin test of the terminals within the connector should be performed. A pin test is performed by inserting the proper size terminal test tool (not a paper clip), into the terminal to determine whether or not the terminal is making good contact, or whether it has been damaged from prior improper connection, multiple connections or lack of connection.

Note; multiple codes can set for just one fault, as if the other codes wanted to come to the party. On an OBD2 system use the Tech 2 scan tool to determine which code set first and under what condition, A DTC17 (370) can trip a DTC18 (P0251), a DTC18 can trip a 19 (P0335) and a DTC19 can trip a DTC35 (P1216) or DTC36 (P

DTC17 (P0370) high resolution circuit fault can be caused by aeration in the fuel from a plugged fuel filter or low fuel supply pump pressure.

DTC18 (P0251) Pump cam reference error

DTC19 (P0335) crankshaft position reference error

DTC34 (P0216) injection timing stepper motor, watch the stepper motor and make sure it retracts when activating TDC learn.

DTC35 (P1216) fuel solenoid response time too short, less than 1.2 ms or less than .75ms on later years.

DTC36 (P1217) fuel solenoid response time too long, more that 2.5 ms, can be caused by a weak fuel control solenoid.

DTC56 (P1218) injection pump calibration resistor error. Usually is caused by a missing resistor, and this codes sometimes only sets after attempting TDC offset learn.

DTC78 (P0236) turbo boost performance, this is usually caused by a bad wastegate control solenoid or low vacuum.

DTC84 (P1125) APP can set if foot is on throttle and brake when key is turned on.

DTC88 (P1214) TDC offset error, the pump to engine timing must be within plus/minus 2 or 2.5 degrees when doing TDC offset learn or this code will set. This code could also set if the starter was used to crank the engine over while installing the injection pump, this will sometimes partially shear the crankshaft timing chain gear key.

1982-1993 6.2L and 6.5L Duramax Diagnostics

Hard Start Hot (after sitting 10 minutes to 2 hours)
1. 6.2L and 6.5L engines need a minimum of 200rpm hot cranking speed (the fan should be a blur). If cranking speed is too slow, they are hard to start (between 120° - 180° coolant temperature) hot, but start fine cold. Check battery connections, batteries, cables and starters. Fix a slow cranking problem first, then if it still starts hard, it could be a problem with the injection pump.

Surges, Runs Rough
1. Check fuel supply pump pressure, it should be about 5-6 PSI on the mechanical supply pump or 4-5 PSI on the electric fuel supply pump. 2. Check for air; use a clear line on the return fitting of the injection pump and look for air. If you have air coming out of the return line then install a clear line before the supply pump.