Throttle Position - Voltage Low
The CAT-SPN51-FMI4 (Caterpillar) diesel fault code means: Throttle Position - Voltage Low. This is a serious severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- advanced
- Estimated cost
- $400-$3500
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Common Symptoms
- Check Engine Light
- Affected sensor reads pegged or default on CAT ET
- System may run in a limp / default mode
Probable Causes (Ranked by Likelihood)
- Sensor / actuator internal failure Likely
- Wiring or connector issue in the affected circuit Likely
- Related system mechanical issue Possible
- ECM calibration or software issue Less Likely
Step-by-Step Diagnostic Procedure
Connect CAT ET and confirm the SPN/FMI is active; record live data for the affected parameter.
Inspect wiring and connectors for the affected circuit; back-probe and check with a DMM.
Perform component test on the sensor / actuator per CAT service manual.
Repair wiring, replace component, or reflash ECM as CAT ET diagnostics direct.
Frequently Asked Questions
What does CAT SPN51-FMI4 mean?
CAT SPN51-FMI4 (Throttle Position - Voltage Low) is a serious-severity fault reported over the J1939 data link. Throttle position below normal range. CAT ECMs report many of the same faults both as CID/FMI (proprietary) and SPN/FMI (J1939) — either form points to the same physical issue.
Can I drive with CAT SPN51-FMI4?
Short distances may be acceptable, but plan the repair promptly. Ignoring the fault risks progressive damage or derate.
How much does CAT SPN51-FMI4 cost to fix?
Typical repair is $400-$3500. Simple sensor/wiring repairs fall at the low end; component, actuator, or aftertreatment work falls at the high end and usually requires CAT ET for programming.
Sources
This page is built from documented references. Verify against your own service info before repair work.