Decode diesel fault codes in plain English
Free J1939 SPN/FMI explainer for trucks, tractors and heavy equipment. What it means, how serious it is, what to check first — and where to get the official manual.
Try SPN 3251 FMI 0, 110-16, FMI 4 or CAT 299D3.
Popular codes
- SPN 3251Aftertreatment 1 DPF Differential Pressure
- SPN 3719Aftertreatment 1 DPF Soot Load Percent
- SPN 4364Aftertreatment 1 SCR Conversion Efficiency
- SPN 5246Aftertreatment SCR Operator Inducement Severity
- SPN 157Injector Metering Rail 1 Pressure (Fuel Rail)
- SPN 100Engine Oil Pressure
- SPN 110Engine Coolant Temperature
- SPN 102Intake Manifold #1 Pressure (Boost)
- SPN 3226Aftertreatment 1 Outlet NOx
- SPN 91Accelerator Pedal Position 1
- SPN 520Actual Retarder – Percent Torque
- SPN 1569Engine Protection Torque Derate
Troubleshooting guides
- DEF/SCR derateAftertreatment / SCR
- NOx sensor failureAftertreatment / SCR
- DPF regen problemsAftertreatment / DPF
- EGR cooler coolant lossEGR / Cooling
- VGT / low boostAir intake / Turbo
- Low rail pressure / air in fuelFuel
- Injector balance rates / misfireFuel / Injectors
- High crankcase pressureEngine / Crankcase ventilation
How to read a J1939 code
A J1939 fault has two parts. The SPN (Suspect Parameter Number) says what is affected — a sensor, a system, a value. The FMI (Failure Mode Identifier, 0–31) says how it failed: too high, too low, shorted, open, erratic, and so on. See all FMIs.