Competitors lock fault-code data behind paid repair subscriptions. We publish it — every code page explains what the code means, what causes it (ranked by probability), how to diagnose it, and which tools read and clear it. Free, forever, no account.
Browse by engine family
Each page decodes that platform's format quirks — Volvo's MID/PID, CAT's flash codes, MaxxForce's dual-network behavior — and links to full diagnostic pages for the most-searched codes.
Understand the format first
Most modern trucks report faults as SPN + FMI: the Suspect Parameter Number identifies what failed, the Failure Mode Identifier how it failed. SPN 3216 FMI 4 means 'NOx sensor signal, voltage below normal.' Five minutes with the SPN/FMI explainer makes every code list on the internet readable.
Legacy systems speak other languages: CAT flash codes, Allison two-digit shifter codes, WABCO blink codes. The per-brand pages translate those too.
The codes that actually sideline trucks
Derate and aftertreatment codes are the #1 roadside panic search — the truck drops to 5 MPH and the driver needs answers now, not a subscription login. Our aftertreatment fault-code page covers SCR, DEF, DPF and derate sequences for every major platform, with the correct recovery procedures.
Fault-code pages by platform
Frequently Asked Questions
What does SPN and FMI mean on a truck scanner?
SPN (Suspect Parameter Number) identifies the failing component or circuit; FMI (Failure Mode Identifier) describes the failure type — voltage high/low, data erratic, mechanical failure etc. Together they form the J1939 fault code. Full explainer: SPN FMI lookup.
Can I clear a derate code without a scanner?
Some derates clear after the underlying fault is fixed and the truck completes a drive cycle; many require a forced regen or a manual reset with a scan tool. Driving on a derate risks a 5-MPH lockout — the aftertreatment page explains recovery per platform.
Why do different scanners show different code numbers for the same fault?
Manufacturers layer proprietary codes over J1939 SPN/FMI. Volvo shows MID/PID, CAT shows flash codes, and some tools translate while others show raw values. Our per-brand pages provide the cross-reference.