Truck Fault Codes: J1939 SPN

Look up any J1939 or J1587 truck fault code offline: SPN, FMI, PID, SID or raw DTC. See what the code means, the likely causes and what to check first - in plain workshop language.

Truck Fault Codes is a free diagnostic code reference for semi trucks, heavy-duty diesel trucks, buses and trailers. Type the error code your dash or scan tool shows, and get a straight answer, even with no signal.

HOW IT WORKS
1. Enter the SPN and FMI (for example SPN 3226 FMI 2), a J1587 MID/PID/SID code or the four raw DTC bytes.
2. Read what the parameter measures and what the failure mode means.
3. Follow the likely causes and the check-first steps, and see if the truck may derate.

WHAT IS INSIDE
- 8000+ J1939 SPN entries with causes and check-first steps
- All 32 J1939 failure modes (FMI) and the 16-mode J1587 table
- J1587 / J1708 lookup by MID + PID or SID + FMI, for older trucks
- Raw DTC byte decoder: four bytes to SPN, FMI and occurrence count
- PGN reference, including DM1 active and DM2 stored codes
- Source addresses: which module is reporting the fault
- Dash lamp meanings: amber, red, MIL and protect
- Symptom-first search: white smoke, no start, derate, low power, DPF, DEF
- Guides on derates, DPF regeneration, DEF/AdBlue and SCR, active vs stored codes

SYSTEMS COVERED
Engine, fuel and air, cooling, aftertreatment (DPF, SCR, EGR, DEF/AdBlue, NOx sensors), transmission, ABS and EBS brakes, electrical and body controllers.

WORKS WITH ANY J1939 TRUCK
J1939 and J1587 are the standard diagnostic languages of heavy vehicles, so the codes apply to trucks and engines such as Volvo, Scania, DAF, MAN, Mercedes-Benz, Iveco, Renault, Freightliner, Kenworth, Peterbilt, International, Mack, Cummins, Detroit, PACCAR and others.

WHO IT IS FOR
Truck drivers and owner-operators on the road, diesel mechanics and technicians, fleet managers and roadside service.

WORKS WITH NO SIGNAL
Everything is on the phone. No account, no sign-in, no download on first use. It works in a yard, under a trailer, or on the hard shoulder with one bar.

40+ LANGUAGES
Explanations and the interface follow the language you pick, not just the menus.

BUILT FOR A WORKSHOP
Adjustable text size and a bold contrast mode, because this gets read in a dark bay with dirty hands.

WHAT IT DOES NOT DO
This is not a scan tool. It does not connect to your truck, it does not read codes from the ECU and it does not clear anything. You enter the code your dash or scan tool gave you, and the app explains it.

An independent reference, not affiliated with, endorsed by or sponsored by any vehicle or engine manufacturer. Brand names are used only to describe compatibility and belong to their owners. Always follow the manufacturer documentation for your vehicle, especially when a warning lamp indicates that the vehicle should be stopped.