Diesel Truck Symptoms — Find Fault Codes by Symptom
Not sure what fault code you have? Browse by symptom to find the most common SPN/FMI codes associated with each symptom, with full diagnostic procedures.
Truck Derate
Engine limiting power or speed. Most common: DEF/SCR faults, DPF soot load, fuel rail pressure.
Check Engine Light
MIL lamp on. Covers all common fault codes that trigger the check engine light.
Low Power
Engine lacks power under load. Most common: low boost, VGT fault, fuel restriction.
Black Smoke
Black exhaust smoke. Most common: low boost, VGT carbon buildup, EGR fault.
DEF System Fault
DEF/SCR warning. Covers all DEF quality, level, heater, and SCR efficiency faults.
Engine Overheating
Coolant temperature high. Most common: fan clutch, thermostat, coolant leak.
Hard Start / No Start
Engine cranks but won't start or starts hard. Covers fuel, cam/crank, and electrical faults.
Rough Idle
Engine runs rough at idle. Most common: injector faults, EGR fault, cam/crank sensor.
Know Your Fault Code?
If you already have a SPN/FMI code, look it up directly in the knowledge base or start an AI diagnostic session for step-by-step repair guidance.