Diesel Repair Blog
Long-form field guides for heavy-duty trucks — written for drivers and shop techs who need practical answers. Looking for a specific code? Search the diesel fault code library.
Showing 1–12 of 119 articles
Compliance
CARB Clean Truck Check 2026: How to Pass the OBD Test the First Time
In 2026 nearly every diesel truck over 14,000 lb GVWR operating in California needs two passing emissions tests per year. A truck that drives fine still fails if the MIL is commanded on, readiness monitors are incomplete, or the ECM won't communicate.
2026-08-29
Engine
Cummins Injector Cup Failure: Coolant-in-Fuel Symptoms and Confirmation Tests
Coolant loss with no external leak plus coolant in the fuel filter points at an injector cup, not a head gasket. Confirm by pressurizing the cooling system cold and watching the fuel filter and cylinders before anyone pulls a head.
2026-08-29
Emissions
DEF in the Diesel Tank (or Diesel in the DEF Tank): Damage Control Steps
Do not start the engine. DEF is 32.5% urea in water and corrodes fuel system metals; diesel destroys SCR catalyst coating and doser seals. Cranking converts a drain-and-flush job into a fuel system or aftertreatment replacement.
2026-08-29
Fuel System
Diesel Fuel Gelling: Cold-Flow Failures, Anti-Gel Timing, and Roadside Recovery
Gelling is paraffin wax dropping out of solution and blocking the fuel filter. Anti-gel additive only works when dosed above the fuel's cloud point — once wax has formed, the only reliable fix is heat, filter replacement, and time.
2026-08-29
Maintenance
Diesel Oil Analysis for Fleets: Reading Soot, Fuel Dilution, and Wear Metals
One oil sample tells you almost nothing; three from the same engine tell you what is wearing. Read viscosity, soot, fuel dilution, glycol, silicon, and wear metals as trends against consistent sampling intervals, not against a single pass or fail.
2026-08-29
Compliance
EPA 2027 NOx Amendments: What the Proposed End of DEF Derates Means
EPA's 2027 NOx limit of 0.035 g/hp-hr stays. What the July 2026 proposal would change is the compliance machinery around it: shorter emissions warranties, useful life delayed to 2030, and DEF-related speed derates replaced by dashboard warnings.
2026-08-29
Drivetrain
Eaton AMT Fault Codes: Air, Voltage, and Clutch Checks Before Teardown
Most Eaton automated transmission faults are not gearbox failures. Verify air supply pressure and dryness, charging system voltage, clutch actuator adaptation, and neutral/park interlocks before anyone quotes a transmission removal.
2026-08-29
Diagnostics
J1939 Datalink Faults: Diagnosing CAN Bus Errors Without Guessing
SPN 639 means the J1939 datalink itself is failing, not the component named in the code. Measure termination resistance and bus voltage before replacing any module — most datalink faults are wiring, connectors, or a single noisy node.
2026-08-29
Fleet Operations
Telematics Fault Alerts: How to Triage Remote Codes Without Chasing False Alarms
A remote fault alert tells you a code set, not that a repair is needed. Triage on lamp severity, active versus inactive status, occurrence count trend, and whether the code is a root cause or a downstream result before dispatching anyone.
2026-08-29
Emissions
Wet Stacking and Idle-Heavy Duty Cycles: Why Low Load Kills Aftertreatment
Aftertreatment needs exhaust heat it cannot make at idle. Trucks that spend most of their hours idling or on short low-load runs accumulate soot faster than they can burn it, dilute their oil with fuel, and fail regens for reasons no part replacement fixes.
2026-08-29
Engine
Cummins ISB 6.7 No-Start: Fuel Pressure Checks Before Sensor Panic
ISB 6.7 no-starts are often fuel delivery, not ECMs. Verify lift pump supply, rail pressure build, and sensor agreement before swapping computers.
2026-08-27
Diagnostics
DPF Differential Pressure Sensor Failure: Symptoms and Tests
A failed DPF differential pressure sensor skews soot logic and regen requests. Prove hoses and sensor voltage before you pull the filter.
2026-08-27
Showing 1–12 of 119 articles