Diesel Diagnostic Library
1,830+ SPN/FMI fault codes, OBD-II codes, system diagnostics, and YMM-specific repair paths for heavy duty and diesel pickup trucks. Every page includes step-by-step diagnostic procedures, torque specs, and verified repair paths.
SPN / FMI Fault Codes
J1939 Suspect Parameter Numbers and Failure Mode Identifiers for Class 6–8 commercial trucks. Used by Detroit, Cummins, PACCAR, Volvo/Mack, and all major OEMs.
SPN 3251, 3936, 4364, 5246 and all aftertreatment fault codes
SPN 411, 412, 2659, 2791 and all EGR/DEF fault codes
SPN 94, 157, 1239, 1347 and all fuel system fault codes
J1939 SPN/FMI codes for Class 6–8 commercial trucks
Fault codes for 6.7 Cummins, 6.7 Powerstroke, L5P Duramax
Most-Searched SPN Codes
FMI Reference — All Failure Mode Identifiers
The FMI tells you how a parameter failed. Combined with the SPN, it pinpoints the exact fault type. Use the SPN/FMI Decoder to look up any combination.
Sensor or parameter reading above normal operating range
Sensor or parameter reading below normal operating range
Signal is unstable, intermittent, or inconsistent
Circuit voltage higher than expected — short to power
Circuit voltage lower than expected — short to ground
Circuit current too low — open circuit or broken wire
Circuit current too high — short to ground
Actuator commanded but not responding as expected
Signal frequency or duty cycle out of range
Data not updating at expected rate — network timeout
Parameter changing too fast or too slow
Fault detected but root cause cannot be isolated
Module or smart sensor has failed internally
Component needs calibration or relearn procedure
Refer to OEM service manual for specific procedure
Parameter moderately below normal — less severe than FMI 1
A condition is present — not necessarily a circuit fault
OBD-II Fault Codes
SAE J1979 standardized codes for light and medium duty diesel vehicles. Applies to 6.7 Cummins, 6.7 Powerstroke, L5P Duramax, and all OBD-II compliant diesel engines.
Most-Searched OBD-II Codes
System Diagnostics
Deep-dive diagnostic guides organized by engine system. Each section covers the full diagnostic path — from symptom to verified repair.
Diesel particulate filter diagnostics, regen failures, ash loading
View guidesSelective catalytic reduction, DEF quality, doser faults
View guidesInlet and outlet NOx sensor diagnostics and replacement
View guidesVGT actuator, boost pressure, turbo oil supply diagnostics
View guidesHigh-pressure fuel rail, CP4, injector contribution diagnostics
View guidesJ1939 network, CAN bus, module communication diagnostics
View guidesEngine Platform Diagnostics
Platform-specific fault codes and repair paths. Select your engine to filter the library to codes and procedures relevant to your truck.
Repair Procedures & Torque Specs
Step-by-step repair procedures with torque specifications for the most common diesel repairs — injectors, EGR, turbochargers, DPF, and more. Each procedure includes torque spec links, sensor check links, and fault code cross-references.
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