Diesel Diagnostics
SPN & FMI Diesel Fault Codes
SPN and FMI fault code lookup for heavy-duty diesel trucks. What each code means, the common causes, what to test first, and whether the truck is safe to drive. Every entry here was written from shop-floor diagnosis, not copied off a code list.
How to read an SPN/FMI code
A J1939 fault is two numbers. The SPN names the parameter the module is watching — a sensor, a circuit, a pressure, a temperature. The FMI names the failure mode: shorted high, shorted low, open circuit, erratic data, not plausible, above or below normal range. Same SPN with a different FMI is a different repair. Read both before you touch a wrench, and pull the whole code set so you know which fault came first.
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All makes fault codes
- SPN 46 FMI 1Air system pressure critically low
- SPN 84 FMI 19Wheel speed data missing
- SPN 98 FMI 1Engine oil level low
- SPN 168 FMI 1Battery voltage low
- SPN 177 FMI 0Transmission oil temperature too high
- SPN 191 FMI 2Transmission output speed erratic
- SPN 629 FMI 12Engine controller internal fault
- SPN 639 FMI 2J1939 data link error
- SPN 1231 FMI 9Missing message from the transmission
- SPN 1761 FMI 1DEF tank level low
International fault codes
- SPN 51 FMI 7EGR throttle valve not responding
- SPN 100 FMI 18Oil pressure below critical
- SPN 111 FMI 1Coolant level low
- SPN 1172 FMI 4Turbo inlet temperature sensor low
- SPN 3216 FMI 20Inlet NOx sensor drifted
- SPN 3251 FMI 16DPF restriction above normal
- SPN 3510 FMI 3Sensor supply voltage 1 high
- SPN 5246 FMI 15SCR inducement level 1
Volvo fault codes
- SPN 84 FMI 2Vehicle speed sensor erratic
- SPN 100 FMI 1Engine oil pressure low
- SPN 110 FMI 0Coolant temperature above limit
- SPN 157 FMI 18Injector rail pressure below spec
- SPN 411 FMI 3EGR differential pressure sensor high
- SPN 1127 FMI 16Turbo boost pressure high
- SPN 2791 FMI 7EGR valve not responding
- SPN 3226 FMI 20Outlet NOx sensor drift
- SPN 3251 FMI 0DPF outlet pressure extremely high
- SPN 3364 FMI 2DEF quality below threshold
- SPN 3719 FMI 16DPF soot load high
- SPN 5246 FMI 0Aftertreatment SCR operator inducement severe
CAT fault codes
- SPN 91 FMI 3Throttle position sensor high
- SPN 94 FMI 1Fuel pressure low
- SPN 100 FMI 1Low engine oil pressure
- SPN 105 FMI 0Intake manifold air temperature high
- SPN 110 FMI 0High coolant temperature
- SPN 172 FMI 3Intake air temperature sensor circuit high
- SPN 1636 FMI 16Intake manifold temperature above warning
PACCAR fault codes
- SPN 94 FMI 1Fuel rail pressure too low
- SPN 100 FMI 1Engine oil pressure critically low
- SPN 102 FMI 3Boost pressure sensor circuit high
- SPN 110 FMI 0Engine coolant temperature too high
- SPN 597 FMI 2Brake switch circuit fault
- SPN 1692 FMI 18Turbocharger underboost
- SPN 2659 FMI 7Intake air heater performance
- SPN 3251 FMI 0DPF differential pressure very high
- SPN 3364 FMI 2DEF quality poor
- SPN 3556 FMI 7Hydrocarbon doser performance
- SPN 3719 FMI 16Soot load high (level 1)
- SPN 3936 FMI 31Aftertreatment system incomplete regeneration
Mack fault codes
- SPN 94 FMI 17Fuel delivery pressure low
- SPN 108 FMI 2Barometric pressure sensor erratic
- SPN 171 FMI 4Ambient air temperature sensor low
- SPN 190 FMI 0Engine overspeed
- SPN 651 FMI 5Injector 1 circuit open
- SPN 3251 FMI 2DPF pressure data erratic with high restriction
- SPN 4364 FMI 18SCR conversion efficiency low
Fault code questions
- What does SPN mean on a diesel fault code?
- SPN stands for Suspect Parameter Number. It is the J1939 number that identifies which component, circuit or measured value the control module is unhappy with — for example SPN 3251 is DPF differential pressure.
- What does FMI mean?
- FMI stands for Failure Mode Identifier. It describes how the value failed: shorted high, shorted low, open circuit, data erratic, data not plausible, or above/below normal range. The FMI is what tells you whether you are chasing wiring or a real physical condition.
- Is a fault code enough to order a part?
- No. A code names a circuit, not a failed part. Verify the reading with live data and a meter before anything gets replaced. That is the whole reason this reference lists tests instead of parts.
Engine-specific lookups
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