Diesel Diagnostics
Diesel Diagnostics
Troubleshooting guides for heavy-duty diesel engines — how to read a fault code, why a DPF won't regen, what triggers a derate, and how to find electrical faults instead of replacing parts. Written from the shop floor, not from a brochure.
How to Diagnose a Diesel Fault Code
A working technician's order of operations for diesel fault codes: read the whole code set, find the first-in code, verify the circuit, and test before you replace anything.
Read the guideDPF Regeneration Problems: Why Your Truck Won't Regen
Why a diesel particulate filter won't regen: inhibit conditions, soot vs ash loading, failed doser and DOC, intake leaks, and the tests that tell you whether the DPF is actually the problem.
Read the guideDEF and SCR Problems: Diagnosing the Dosing System
Diagnose DEF and SCR faults on a heavy-duty diesel: dosing pressure, crystallization, DEF quality and level sensors, NOx sensor disagreement, and low SCR conversion efficiency.
Read the guideEngine Derate Troubleshooting: Why a Diesel Loses Power
What triggers a heavy-duty diesel derate, how the staged derate ladder works, and how to find the fault behind the power loss — emissions, protection, and mechanical causes.
Read the guideDiesel No-Start Diagnostics: Cranks But Won't Start
A structured no-start diagnosis for heavy-duty diesels: crank speed and voltage, fuel supply and rail pressure, air intake restriction, and the ECM inputs that block a start.
Read the guideDiesel Electrical Diagnostics: Grounds, 5V Circuits, and J1939
How to diagnose heavy-truck electrical faults: voltage drop testing, shared 5-volt reference circuits, corroded grounds, and J1939 data bus troubleshooting.
Read the guideCummins ISX15 DPF and Regen Problems
Cummins ISX15 aftertreatment troubleshooting: repeated regens, high soot load, delta pressure faults, EGR and VGT causes, and what to test before replacing the filter.
Read the guidePACCAR MX-13 Fault Codes: How to Read and Diagnose Them
How PACCAR MX-13 fault codes are structured, the systems that generate most of them, and a diagnostic order that avoids replacing good aftertreatment parts.
Read the guideLook a code up, or let the AI walk it with you
- Cummins ISX fault code lookup
- Detroit DD15 fault code lookup
- Blue Collar AI diesel diagnostic support
- Aftertreatment Mastermind DPF/SCR training
All of it is in the Truck Commander membership — $14.99 a month.