Understanding Truck Fault Code Formats
Before diving into specific codes, it helps to understand the format. Heavy-duty trucks use the J1939 protocol which reports faults as SPN/FMI pairs:
- SPN (Suspect Parameter Number): Identifies the component or system (e.g., SPN 100 = engine oil pressure)
- FMI (Failure Mode Identifier): Describes what went wrong (e.g., FMI 1 = data valid but below normal range)
Some trucks also display SA (Source Address) which identifies which module reported the fault (e.g., SA 0 = engine, SA 33 = body controller).
The Top 20 Fault Codes
Engine & Fuel System
1.SPN 100 / FMI 1 — Engine Oil Pressure Low
- Check oil level immediately. Could be low oil, failed oil pump, clogged filter, or faulty sensor.
- 🛑 Stop driving if oil pressure gauge confirms low pressure. Engine damage can occur within minutes.
2.SPN 110 / FMI 0 — Engine Coolant Temperature High
- Overheating. Check coolant level, radiator for blockage, fan clutch operation, thermostat.
- ⚠️ Pull over safely and let the engine cool before opening the radiator cap.
3.SPN 91 / FMI 4 — Accelerator Pedal Position Sensor Voltage Low
- Throttle position sensor issue. Check wiring harness and connector at the pedal. Common after cab work.
4.SPN 190 / FMI 0 — Engine Overspeed
- Engine exceeded maximum RPM. Usually caused by downhill runaway, turbo failure, or transmission issue.
5.SPN 157 / FMI 18 — Fuel Rail Pressure Low
- Low fuel pressure at the common rail. Check fuel filters, fuel supply pump, rail pressure sensor, and look for air leaks in fuel lines.
Aftertreatment / Emissions
6.SPN 3251 / FMI 0 — DPF Soot Load Too High
- Needs a forced regen. See our DPF Regeneration Guide for full procedure.
7.SPN 3364 / FMI 18 — DEF Dosing Malfunction
- The DEF injection system isn't working properly. Check DEF level, DEF quality, and the dosing valve for crystallization.
8.SPN 3226 / FMI 3 — SCR NOx Conversion Efficiency Low
- The SCR catalyst isn't reducing NOx effectively. Could be bad DEF, failed SCR catalyst, or NOx sensor issue.
9.SPN 5246 / FMI 0 — Aftertreatment SCR Operator Inducement (Derate)
- Emissions system has triggered a power derate. Typically a 25% then 40% derate with a countdown timer.
10.SPN 4364 / FMI 18 — Aftertreatment DEF Tank Level Low
- Fill the DEF tank. If the level reads low after filling, the DEF level sensor may be faulty.
Electrical & Sensors
11.SPN 627 / FMI 14 — ECM/ECU Communication Error
- The engine control module lost communication. Check battery connections, ground straps, and CAN bus wiring.
12.SPN 168 / FMI 1 — Battery Voltage Low
- Charging system issue. Check alternator output, battery condition, cable connections.
13.SPN 639 / FMI 2 — J1939 Data Link Error
- Communication bus problem. Multiple modules may lose communication. Check CAN bus terminating resistors.
Turbo & Air System
14.SPN 102 / FMI 0 — Boost Pressure High
- Overboost condition. Could indicate wastegate failure, turbo actuator problem, or MAP sensor issue.
15.SPN 102 / FMI 1 — Boost Pressure Low
- Underboost. Check for boost leaks, turbo failure, charge air cooler leaks, or clogged air filter.
16.SPN 132 / FMI 0 — Mass Air Flow High
- MAF sensor reading too high. Check for sensor contamination or intake leaks.
Transmission & Drivetrain
17.SPN 523 / FMI 14 — Transmission Communication Lost
- ECM lost contact with the transmission controller. Check wiring between engine and transmission ECUs.
18.SPN 597 / FMI 2 — Clutch Switch Error
- Clutch position sensor malfunction. Common on automated manual transmissions.
Brakes
19.SPN 802 / FMI 5 — ABS Wheel Speed Sensor Fault
- Typically a broken or damaged wheel speed sensor wire. Check at the wheel end, especially after brake work.
20.SPN 1569 / FMI 31 — Parking Brake Warning
- Could be low air pressure, failed park brake valve, or wiring issue.
What To Do When You Get a Fault Code
1.Note the code — Write down or photograph the exact SPN/FMI
2.Check severity — Red warning lights = stop. Yellow = proceed with caution
3.Check the basics — Fluid levels, visible damage, loose connectors
4.Clear and monitor — Some codes are intermittent. If it comes back, the problem is real
5.Get to a shop — If you see derate warnings or red stop engine lights