Professional Diagnostic Library
Heavy-Duty Diesel Diagnostics for Drivers and Techs
Look up SPN / FMI / P-codes by brand, then follow the test order shops actually use — plus field guides for DPF, DEF, SCR, and derates.
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
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Browse fault codes by brand
767 diagnostic pages covering SPN / FMI, PID, and OBD-II codes across 24 engine and chassis platforms.
- Ford Power Stroke65
- Cummins60
- GM Duramax57
- Ram Cummins57
- Detroit Diesel55
- All Brands49
- Nissan Titan XD44
- Volvo43
- Mack41
- PACCAR40
- International38
- Freightliner35
- Caterpillar34
- Scania26
- Mercedes-Benz25
- Kenworth24
- Peterbilt24
- Universal Diesel23
- Heavy-Duty13
- OBD-II Diesel10
- Ford1
- GM1
- Nissan1
- Ram1
Most severe codes techs look up
- 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
- Ford Power Stroke P-Code P0217 — Engine Coolant Temperature System
- Cummins SPN 100 FMI 18 — Engine Oil Pressure Sensor
- GM Duramax P-Code P0524 — Engine Oil Pressure
- Ram Cummins P-Code P0093 — Fuel System Large Leak Detected
- Detroit Diesel SPN 111 FMI 1 — Engine Coolant Level Sensor
- All Brands Guide Ash-Soot — DPF Ash and Soot Load Models
- Nissan Titan XD P-Code P0217 — Engine Cooling System
- Volvo SPN 111 FMI 1 — Engine Coolant Level Sensor
- Mack SPN 1569 FMI 31 — Engine Protection Torque Derate
- PACCAR SPN 111 FMI 1 — Engine Coolant Level Sensor
- International SPN 100 FMI 18 — Engine Oil Pressure Sensor
- Freightliner SPN 190 FMI 0 — Engine Speed Sensor Circuit
- Caterpillar SPN 100 FMI 1 — Engine Oil Pressure Sensor
- Scania SPN 100 FMI 1 — Engine Oil Pressure Sensor
- Mercedes-Benz SPN 190 FMI 0 — Engine Speed Sensor Circuit
- Kenworth SPN 110 FMI 0 — Engine Coolant Temperature Sensor
- Peterbilt SPN 111 FMI 1 — Engine Coolant Level Sensor
- Universal Diesel P-code P0088 — Fuel Rail Pressure Too High
- Heavy-Duty SPN 157 FMI 0 — Fuel Rail Pressure
- OBD-II Diesel P-Code P0087 — Fuel Rail Pressure
- Ford Power Stroke P-Code P228D — Fuel Pressure Regulator 1 Exceeded Learning Limit
- Cummins SPN 157 FMI 16 — Engine Fuel Rail Pressure
- GM Duramax P-Code 0093 — Fuel System
- Ram Cummins P-Code P0217 — Engine Cooling System
Latest diagnostic guides
Compliance
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.
Engine
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.
Emissions
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.
Fuel System
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.
Maintenance
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.
Compliance
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.