What this code means

Component / fault: Throttle position.
Reported detail: invalid signal.
FMI 8 in the SAE J1939 / Caterpillar CDL convention means: Abnormal frequency or pulse width or period — see the full FMI reference table.

On a Caterpillar display this code may appear as MID/CID 91 with FMI 8, or in the older PID/SID style. Wording of the reported detail follows the public Caterpillar fault-code listing cited below.

First checks (general, for this failure type)

Signal frequency or pulse is abnormal. Check sensor air gap, tone ring / reluctor damage, and wiring routed near ignition or alternator interference.

These are general electrical checks for FMI 8 faults, not a substitute for the model-specific test procedure. If the fault is active and the machine is derated or shut down, follow the service manual for your model and serial prefix.

Need the part?

If the checks point to a failed sensor, solenoid or harness part, send us the code (91 / FMI 8), the machine model and serial number — we quote genuine and OEM replacement parts for export.

Same component, other failure modes

FMI 13