Fault code lookup
Diesel Fault Code Library
Search SPN/FMI and OBD-II codes by brand and system. Built for shop techs, owner-operators, and fleet managers who need the next test — not a parts list.
Clear SPN/FMI breakdowns and practical next steps. Saved a Cascadia SCR call from turning into a parts-guessing session.
Marcus R. · Fleet technician
Easy to search by brand and code. The derate guidance helped me decide whether to limp or call a tow.
Dana K. · Owner-operator
Solid reference for common Cummins and Detroit codes. We bookmark fault pages for apprentices during intake.
Chris P. · Shop lead
Loading fault code library…
How to read a truck fault code
SPN
Suspect Parameter Number — the component or measurement (DEF quality, NOx, rail pressure). Same SPN can appear on Cummins, Detroit, PACCAR, and Volvo.
FMI
Failure Mode Identifier — how it failed (high voltage, data erratic, below normal). FMI 3/4 are usually electrical; FMI 18 is often a real process value.
P-code
OBD-II style codes on pickups and some medium-duty chassis (P0087, P0299, P0401). Search either format — both resolve to the same diagnostic page when we have it.
Full walkthrough: SPN vs FMI for drivers. If a cheap scanner only shows a result code like 5246, start here: SPN 5246 is not the root fault.
Fault codes by brand
Each brand hub lists that platform's codes grouped by system (aftertreatment, fuel, air/turbo, coolant, sensors). Open the hub for your engine — that is the crawlable index, not a 700-link dump on this page.
- Ford Power Stroke65 codes
- Cummins60 codes
- GM Duramax57 codes
- Ram Cummins57 codes
- Detroit Diesel55 codes
- All Brands49 codes
- Nissan Titan XD44 codes
- Volvo43 codes
- Mack41 codes
- PACCAR40 codes
- International38 codes
- Freightliner35 codes
- Caterpillar34 codes
- Scania26 codes
- Mercedes-Benz25 codes
- Kenworth24 codes
- Peterbilt24 codes
- Universal Diesel23 codes
- Heavy-Duty13 codes
- OBD-II Diesel10 codes
- Ford1 codes
- GM1 codes
- Nissan1 codes
- Ram1 codes
High-severity codes techs look up first
- Ford Power Stroke Guide CP4-67PS — High-Pressure Fuel Pump (CP4)
- Cummins SPN 100 FMI 1 — Engine Oil Pressure Sensor / Oil Pressure Circuit
- GM Duramax P-Code P0217 — Engine Coolant Over Temperature Condition
- Ram Cummins P-Code 0606 — ECM/PCM
- Detroit Diesel SPN 100 FMI 1 — Engine Oil Pressure Sensor
- All Brands Guide DEF-Limp — DEF Inducement 5 MPH Limp Mode
- Nissan Titan XD P-Code P2BAD — NOx Concentration Exceeded (Bank 1)
- Volvo SPN 100 FMI 1 — Engine Oil Pressure
- Mack SPN 190 FMI 0 — Engine Speed Sensor Circuit
- PACCAR SPN 100 FMI 1 — Engine Oil Pressure Sensor
- International SPN 100 FMI 1 — Engine Oil Pressure Sensor
- Freightliner SPN 111 FMI 1 — Engine Coolant Level Sensor
- Caterpillar SPN 94 FMI 1 — Fuel Delivery Pressure Sensor
- Scania SPN 94 FMI 1 — Fuel Delivery Pressure Sensor
- Mercedes-Benz SPN 100 FMI 1 — Engine Oil Pressure Sensor / Oil Pressure Circuit
- Kenworth SPN 100 FMI 1 — Engine Oil Pressure Sensor
- Peterbilt SPN 100 FMI 1 — Engine Oil Pressure Sensor / Oil Pressure Circuit
- Universal Diesel P-code P0087 — Fuel Rail Pressure Too Low
- Heavy-Duty SPN 110 FMI 0 — Engine Coolant Temperature
- OBD-II Diesel P-Code P20EE — SCR NOx Catalyst
- Ford Guide P20EE — Ford 6.7 Powerstroke SCR System
- GM OBD-II P20BA — DEF Tank Heater
- Nissan OBD-II P20EE — SCR Catalyst System
- Ram Guide P20E8 — Ram 6.7 Cummins DEF Pump System
Related diagnostic guides
CARB Clean Truck Check 2026: How to Pass the OBD Test the First Time
In 2026 nearly every diesel truck over 14,000 lb GVWR operating in California needs two passing emissions tests per year. A truck that drives fine still fails if the MIL is commanded on, readiness monitors are incomplete, or the ECM won't communicate.
Cummins Injector Cup Failure: Coolant-in-Fuel Symptoms and Confirmation Tests
Coolant loss with no external leak plus coolant in the fuel filter points at an injector cup, not a head gasket. Confirm by pressurizing the cooling system cold and watching the fuel filter and cylinders before anyone pulls a head.
DEF in the Diesel Tank (or Diesel in the DEF Tank): Damage Control Steps
Do not start the engine. DEF is 32.5% urea in water and corrodes fuel system metals; diesel destroys SCR catalyst coating and doser seals. Cranking converts a drain-and-flush job into a fuel system or aftertreatment replacement.
Diesel Fuel Gelling: Cold-Flow Failures, Anti-Gel Timing, and Roadside Recovery
Gelling is paraffin wax dropping out of solution and blocking the fuel filter. Anti-gel additive only works when dosed above the fuel's cloud point — once wax has formed, the only reliable fix is heat, filter replacement, and time.
Diesel Oil Analysis for Fleets: Reading Soot, Fuel Dilution, and Wear Metals
One oil sample tells you almost nothing; three from the same engine tell you what is wearing. Read viscosity, soot, fuel dilution, glycol, silicon, and wear metals as trends against consistent sampling intervals, not against a single pass or fail.
EPA 2027 NOx Amendments: What the Proposed End of DEF Derates Means
EPA's 2027 NOx limit of 0.035 g/hp-hr stays. What the July 2026 proposal would change is the compliance machinery around it: shorter emissions warranties, useful life delayed to 2030, and DEF-related speed derates replaced by dashboard warnings.