Truck fault codes
15 codes documented. Each one carries what the code means, what it usually turns out to be, how a technician isolates it, what the parts and labour run, and whether you can keep driving.
Have the number in front of you? Use the lookup — SPN and FMI in two fields, or paste a P-code.
J1939 (heavy duty)
SPN and FMI pairs from the engine, aftertreatment, and chassis networks on Class 6–8 equipment.
- SPN 46 FMI 1Out of servicein reviewPneumatic Supply Pressure — Below Normal, Most SevereAir system pressure is below the level the brakes need. This is a brake system fault, not an engine fault. Do not move the truck on the theory that it will build back up.
- SPN 168 FMI 1Cautionin reviewBattery Potential / Power Input 1 — Below Normal, Most SevereSystem voltage is below what the electronics need. Left alone this ends with a truck that will not restart — and on a modern truck, low voltage also throws phantom fault codes all over the bus.
- SPN 1569 FMI 31Derate riskin reviewEngine Protection Torque Derate — Condition ExistsThis code is not a fault. It is the truck telling you it has already cut power because of something else. Find the other code — that is the repair.
- SPN 3216 FMI 16Cautionin reviewSCR Inlet NOx Sensor — Reading Above Normal RangeThe NOx sensor before the SCR catalyst is reporting more NOx than the engine should be making. Usually a drifting sensor or an EGR problem upstream — not the catalyst itself.
- SPN 3226 FMI 16Cautionin reviewSCR Outlet NOx Sensor — Reading Above Normal RangeToo much NOx is getting past the SCR catalyst. Either the system is not dosing enough DEF, the catalyst has lost efficiency, or the outlet sensor is lying. All three are worth catching now.
- SPN 3251 FMI 0Derate riskin reviewDPF Differential Pressure — Above Normal, Most SevereBackpressure across the particulate filter is high enough that the engine is being protected. Either the filter is genuinely loaded, or the pressure sensor and its tubes are lying to you.
- SPN 3364 FMI 2Derate riskin reviewDEF Quality / Concentration — Data Erratic or IncorrectThe truck thinks your DEF is the wrong concentration or the quality sensor is giving impossible readings. It is usually the DEF or the sensor, not the engine. Fix it before the inducement clock runs out.
- SPN 3719 FMI 0Derate riskin reviewDPF Soot Load — Above Normal, Most SevereSoot load has passed the point where a normal regeneration will recover it. Do not keep driving hoping it burns off — at this level a forced regen can damage the filter.
- SPN 4364 FMI 18Derate riskin reviewSCR Conversion Efficiency — Below Normal, Moderately SevereThe catalyst is not converting enough NOx to meet the certified standard. This is the fault that starts the inducement countdown — it does not clear itself and it does not wait.
- SPN 5246 FMI 0Out of servicein reviewSCR Operator Inducement — Most Severe Level ActiveThis is not a warning. The emissions system has escalated to full inducement and the truck is about to be limited to a speed you cannot safely use on a public road. Get it somewhere safe now.
OBD-II (light and medium duty)
P-codes on diesel pickups and Class 3–5 trucks. Standardised definitions, engine-specific repairs.
- P20EEDerate riskin reviewSCR NOx Catalyst Efficiency Below Threshold — Bank 1The SCR catalyst is not cleaning up enough NOx. On a diesel pickup this is the code that eventually limits you to a crawl, so the countdown on the dash is real.
- P226CCautionin reviewTurbocharger Boost Control A — Slow ResponseThe variable-geometry turbo is not moving to the commanded position quickly enough. On a 6.7 Power Stroke or an L5P this is usually a sticking vane mechanism or the actuator, not a blown turbo.
- P0402Cautionin reviewEGR Flow Excessive DetectedMore exhaust gas is recirculating than the ECM asked for — usually a valve stuck open with carbon. Expect rough idle, smoke, and worse fuel economy until it is cleaned or replaced.
Thermo King reefer alarms
Alarm codes from the refrigeration unit controller. Urgent by nature — there is a load in the box.
- Alarm Code 10Out of servicein reviewHigh Discharge PressureRefrigerant discharge pressure hit the safety limit. The unit protects itself by shutting down — which means the clock on your load has already started. Check airflow through the condenser first.
- Alarm Code 18Out of servicein reviewHigh Engine Coolant TemperatureThe reefer's diesel engine is overheating. On most units this is a shutdown alarm, so treat it as a live load-temperature problem and not a maintenance item for next week.
By manufacturer
Why this index is short
There are thousands of J1939 SPNs. Generating a page for each one is a weekend of work and produces nothing anybody can use. Our gate is that a code page publishes only when it carries either a credentialed technician's annotation or at least three cited technical sources plus real parts and labour figures — and the gate is enforced in code, so a page that fails it cannot ship.
0 pages have cleared that bar. 15 are written and waiting on technical review; they are readable, labelled, and excluded from our sitemap until they are signed off. Our editorial policy explains the whole gate.