Diesel Fault Code Lookup (SPN/FMI) for Owner Operators

Look up common J1939 SPN and FMI codes — what they mean and what commonly causes them.

This covers the most commonly diagnosed codes — engine core sensors and the DPF/DEF/SCR emissions system, where most real-world shop visits happen. SAE J1939 defines over 10,000 SPNs; this isn't the full catalog. For a code not listed here, check your engine manufacturer's fault code guide or a dealer scan tool.

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Code Detail

Select an SPN on the left to see what it means.

A code tells you what and how something failed. It is not automatically the root cause or the part to replace. Use it to narrow down where to look, then confirm with a proper diagnosis before you order parts.

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How do I read a diesel fault code (SPN/FMI)?

Heavy-duty diesel engines report faults using the SAE J1939 standard: a Suspected Parameter Number (SPN) identifies which component or system triggered the fault, and a Failure Mode Identifier (FMI) describes the type of failure, such as a reading that is too high, too low, or an open circuit. The same SPN can appear with different FMIs for different problems, so read the pair together, not the SPN alone. A derate code (reduced power or speed) means the engine has limited itself to protect the aftertreatment or itself, and should not be ignored.

Read both numbers together: the SPN tells you the system (for example, the DEF system or an NOx sensor), the FMI tells you the kind of fault.
Check whether the code includes a derate or shutdown warning. Those mean the engine has already limited performance and need prompt attention.
Check your engine manufacturer's documentation (Cummins, Detroit, PACCAR, Volvo, International) for the exact meaning of your SPN/FMI pair, since some codes vary slightly by manufacturer.
Note whether the code is active or stored (historical). An active code is the current problem; a stored code may be an intermittent issue.
Decide if it's safe to keep driving based on the manufacturer's guidance for that code, not a guess, especially for any brake, steering or aftertreatment-related fault.

How to read an SPN/FMI pair
Part of the code	What it tells you
SPN (Suspected Parameter Number)	Which component or system reported the fault, for example a sensor or the DEF system
FMI (Failure Mode Identifier)	The type of failure: for example, voltage too high, too low, or an open circuit
Active vs. stored	Whether the fault is happening now or was recorded in the past

SPN/FMI is a shared SAE J1939 standard, but exact code meanings and recommended actions can vary by engine manufacturer. Confirm any code against your specific engine's documentation before deciding whether to keep driving.

Last reviewed October 2026. Sources: SAE International: J1939 digital annex (diagnostic standard).

Frequently Asked Questions - Diesel Fault Code Lookup (SPN/FMI)

What's the difference between an SPN and an FMI?

The SPN (Suspect Parameter Number) identifies which part or parameter has the problem, like engine oil pressure or the DEF dosing unit. The FMI (Failure Mode Identifier) describes how it failed, such as open circuit, voltage too high, or erratic signal. A full fault code is always the two together, like SPN 3364 FMI 5.

Does a fault code tell me exactly what part to replace?

No. It tells you what's being monitored and how the failure behaves electrically. It doesn't automatically name the root cause. SPN 641 FMI 7, for example, points at the turbo wastegate/VGT actuator not responding. That can mean a stuck actuator, carbon buildup, or a wiring issue. Use the code to narrow down where to look, then confirm before you order parts.

Why do DPF and DEF codes trigger a power derate?

EPA rules require diesel emissions systems to force a power or road-speed derate. That happens when the aftertreatment system (DPF, DEF/SCR, or NOx sensors) can't confirm it's working right. It is built so an emissions fault can't be ignored. The derate itself (often reported as SCR Operator Inducement) is a symptom. Fixing the underlying DEF quality, dosing, or sensor fault clears it.

Is this the same fault code list for Cummins, Detroit, and PACCAR engines?

The SPN/FMI system itself (SAE J1939) is a shared industry standard across all major heavy-duty engine brands. So the same SPN means the same parameter on a Cummins X15, Detroit DD15, or PACCAR MX-13. Makers can still add their own codes and dash text on top. So always cross-check anything unusual against your specific engine's service documents.

Why does this tool list only some SPN codes?

The J1939 standard defines thousands of SPNs. This tool lists the ones drivers and shops look up most, mostly core engine sensors and the DPF, DEF and SCR system, and each entry was checked against more than one source. If your SPN is not listed, the FMI meaning still applies, and your engine maker's service documents give the exact name. A code that is not here does not mean the fault is minor.

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Go Further With Diesel Fault Code Lookup (SPN/FMI)

Guides, services and reference pages that go with this tool.

Terms explained in the Diesel Fault Code Lookup (SPN/FMI)

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