Manufacturer Expertise
Chevrolet / GMC Duramax diagnostics, repair & maintenance
STL Diesel works on diesel trucks in Crystal City, Missouri and serves owners and fleets throughout Festus, Herculaneum, Pevely, Hillsboro, De Soto, Jefferson County and the southern St. Louis region.
Our process starts with the customer concern, a vehicle inspection and scan data, followed by system-specific testing. We diagnose before recommending parts so the repair plan is based on test results rather than guessing.
Book an AppointmentEngines & generations we support
- LB7 Duramax
- LLY Duramax
- LBZ Duramax
- LMM Duramax
- LML Duramax
- L5P Duramax
Call 636-465-0585 with your year, model, engine and symptoms.
Common Concerns
What we diagnose on Chevrolet / GMC Duramax trucks
Fuel rail pressure & injectors
Low rail pressure, hard starts, injector balance or return-flow concerns and contaminated fuel require system testing before replacement decisions.
CP4 / high-pressure fuel system
On applications equipped with a CP4, pump and contamination concerns are evaluated with the rest of the high-pressure fuel system in mind.
NOx, DEF & SCR faults
NOx sensor, DEF delivery, SCR efficiency and exhaust-temperature faults can overlap. Codes, commanded data and system tests should guide the repair.
Turbocharger & boost
Charge-air leaks, variable-geometry control, sensor data and exhaust restrictions can produce similar underboost or performance complaints.
Cooling & head-gasket symptoms
Coolant loss or pressure complaints can have multiple causes. Pressure testing and other checks help separate external leaks, cooling-component faults and internal engine concerns.
Allison & drivetrain concerns
Shift quality, temperature, fluid condition and transmission data are evaluated along with engine torque and related drivetrain systems.
How STL Diesel Diagnoses
A test-first repair process
- Confirm the concern. We start with the symptoms, operating conditions, history and stored fault codes.
- Check data and basics. We compare commanded and actual values and inspect the mechanical, electrical, air, fuel and cooling systems involved.
- Perform pinpoint tests. Pressure, leak, electrical, contribution, temperature or other tests are used as appropriate.
- Explain the findings. You receive the test results and repair priorities before work moves forward.
- Verify the repair. The system is rechecked after repair to confirm the original problem is corrected.
Quick Answers
Chevrolet / GMC Duramax repair FAQ
Which Duramax generations does STL Diesel service?
STL Diesel works on LB7, LLY, LBZ, LMM, LML and L5P Duramax trucks. Call with your model year and symptoms for service availability.
What can cause P20EE on a Duramax?
P20EE indicates SCR efficiency below threshold. Possible causes include DEF quality or delivery issues, NOx-sensor problems, exhaust leaks, SCR catalyst concerns or an upstream engine condition that changes emissions.
What does low fuel-rail pressure mean?
It means actual fuel pressure is not meeting the commanded value under a specific condition. Filters, supply, regulators, injector return, high-pressure pump condition and contamination may all need to be checked.
Need help with your Chevrolet / GMC Duramax?
Call STL Diesel at 636-465-0585 or schedule an appointment in Crystal City, Missouri.
Book Appointment