Heavy-Duty Diesel Truck Fault Codes
Decode fault codes from Class 7 and Class 8 on-highway trucks. Covers Cummins ISX and X15, Detroit Diesel DD13 / DD15 / DD16, PACCAR MX-11 and MX-13, CAT C15 ACERT, Volvo D13, and Mack MP8. Includes J1939 SPN/FMI pair decoding, regen / derate conditions, aftertreatment faults, and common shop fixes.
Browse by Engine
Cummins ISX / X15
Cummins ISX and X15 big bore engines: fault codes, SPN/FMI pairs, regen issues, derate conditions.
Cummins ISB 6.7L
Medium-duty Cummins ISB 6.7L fault codes for Ram HD, Freightliner M2, Kenworth T270/T370.
PACCAR MX-11 / MX-13
PACCAR MX-11 and MX-13 fault codes in Kenworth and Peterbilt trucks.
Detroit Diesel DD13 / DD15 / DD16
Detroit Diesel DD-series fault codes for Freightliner Cascadia and Western Star.
Caterpillar C15 / C13 / C7
Legacy CAT on-highway diesel engines: C15 ACERT, C13, C7 fault codes and flash codes.
Volvo D13 / Mack MP8
Volvo D11, D13, D16 and Mack MP7, MP8, MP10 fault codes.
What is a Heavy-Duty Diagnostic Code?
Heavy-duty trucks communicate over the J1939 CAN bus, a protocol defined by SAE for commercial vehicles. Unlike light-duty OBD-II (which uses P/B/C/U codes), J1939 faults are reported as SPN (Suspect Parameter Number) paired with an FMI (Failure Mode Identifier). For example, SPN 3251 FMI 0 describes a DPF outlet pressure above normal operating range. Some manufacturers also emit proprietary "fault codes" (Cummins FC 3712) or flash codes in addition to J1939 SPN/FMI pairs.
Common Heavy-Duty Fault Categories
- Aftertreatment (DPF, DEF, SCR, dosing unit, NOx sensors)
- EGR system (cooler, valve, differential pressure)
- Turbocharger (VGT actuator, boost pressure, overspeed)
- High-pressure fuel system (rail pressure, injectors, CP pump)
- Engine derate conditions (inducement levels 1-4)
- Air system (charge air cooler leaks, intake manifold pressure)
- Transmission (Allison 3000/4000, Eaton Fuller UltraShift, ZF TraXon)