Caterpillar doesn't use the same fault code format as most on-highway trucks. Instead of a single SPN, a CAT code is built from a MID, a CID and an FMI — three numbers that together tell you which module is reporting, which component failed, and how. Once you know how to read that combination, CAT fault codes become straightforward. Here's the complete breakdown.
A Caterpillar code has three parts: the MID (Module Identifier) says which control module reported the fault, the CID (Component Identifier) says which component or circuit is affected, and the FMI (Failure Mode Identifier) says how it failed. For example CID 0091 FMI 08 points to the throttle position sensor (0091) with an abnormal signal (08). You read the full code and its description in CAT ET (Caterpillar Electronic Technician) with a CAT Communication Adapter.
MID, CID and FMI: the three parts of a CAT code
- MID (Module Identifier) — identifies the electronic control module that logged the fault, for example the engine ECM, transmission or machine controller. Useful when a machine has several modules on the data link.
- CID (Component Identifier) — identifies the specific component, sensor or circuit, such as a throttle position sensor or an injector solenoid.
- FMI (Failure Mode Identifier) — describes the type of failure. CAT uses the same FMI convention as J1939, so the numbers carry the same meaning.
For a deeper walkthrough with worked examples, see our guide on how to read Caterpillar fault codes.
Common CAT FMI values
| FMI | Meaning |
|---|---|
| 00 | Data valid but above normal range |
| 01 | Data valid but below normal range |
| 02 | Data erratic, intermittent or incorrect |
| 03 | Voltage above normal or shorted high |
| 04 | Voltage below normal or shorted low |
| 05 | Current below normal or open circuit |
| 06 | Current above normal or grounded circuit |
| 08 | Abnormal frequency, pulse width or period |
| 13 | Out of calibration |
Fault codes vs. event codes on Caterpillar
This trips up a lot of technicians. Caterpillar reports two different things:
- Diagnostic (fault) codes — an electrical/electronic problem with a component or circuit, expressed as MID-CID-FMI.
- Event codes — an abnormal operating condition, such as high coolant temperature or low oil pressure, expressed as an E-code (for example E361 High Engine Coolant Temperature). Event codes are not wiring faults — they mean the machine detected a real operating problem.
Knowing which type you're looking at saves you from chasing a wiring fault that doesn't exist.
Example CAT CID references
These are some commonly referenced Component Identifiers. The exact CID list depends on the engine and machine platform, so confirm the reading in CAT ET for your equipment.
| CID | Component |
|---|---|
| 0001–0008 | Injector / cylinder solenoids |
| 0091 | Throttle position sensor |
| 0100 | Engine oil pressure sensor |
| 0110 | Engine coolant temperature sensor |
| 0164 | Injection actuation pressure |
| 0168 | Electrical system voltage |
How to read Caterpillar codes with CAT ET
- Connect a CAT Communication Adapter III to the machine's service port.
- Open CAT ET (Caterpillar Electronic Technician) and read active and logged diagnostic codes plus event codes.
- Record the MID-CID-FMI (or E-code), review the status parameters, and follow the troubleshooting procedure.
- Repair the cause, then clear logged codes in CAT ET and verify they don't reset.
The Caterpillar package includes CAT ET plus SIS service information and flash files — everything you need to read codes, look up the repair and reflash the ECM, installed remotely on your PC.
View the Caterpillar Package →Read Caterpillar Fault Codes with CAT ET Software
For live MID, CID, FMI and event-code diagnostics, use CAT ET software 2026B or 2026A with a compatible Caterpillar Communication Adapter. CAT SIS is used separately for the applicable troubleshooting procedure and repair information.
Frequently asked questions
What is the difference between a CID and an SPN?
They serve the same purpose — identifying the faulty component — but CID is Caterpillar's identifier used with MID and FMI, while SPN is the J1939 identifier used on most on-highway trucks.
What does an E-code mean on a Caterpillar?
An E-code is an event code — it reports an abnormal operating condition (like high coolant temperature), not an electrical wiring fault. Address the operating condition rather than chasing a circuit.
Do I need a CAT adapter to read the codes?
Yes. CAT ET communicates through a CAT Communication Adapter. For most off-highway CAT equipment the OEM adapter is required; some on-highway CAT truck engines can be reached with a supported RP1210 adapter.
Related guides: How to read Caterpillar fault codes · What is CAT ET?
Brand names are used only to describe compatibility. Always follow the manufacturer's service procedures for your specific engine or machine.