Engine Coolant Temperature Sensor - Voltage Low / Short to Ground
The CAT-CID2-FMI4 (Caterpillar) diesel fault code means: Engine Coolant Temperature Sensor - Voltage Low / Short to Ground. This is a moderate severity code.
- Keep driving?
- Yes, but fix soon
- DIY difficulty
- intermediate
- Estimated cost
- $150-$400
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Common Symptoms
- Check Engine Light
- ECT reads pegged high (typically 260°F+) on CAT ET
- Fan runs constantly
- Possible derate from false overheat indication
Probable Causes (Ranked by Likelihood)
- Signal wire shorted to ground or return Very Likely
- ECT sensor internally shorted Likely
- Moisture / coolant intrusion in sensor connector Possible
Step-by-Step Diagnostic Procedure
Confirm fault with CAT ET; unplug sensor — if reading flips to -40°F, wire is OK, sensor is shorted.
Measure resistance sensor signal to ground with connector off; should be open. Short = harness issue.
Inspect connector for coolant intrusion; dielectric grease and repair.
Replace ECT sensor if proven shorted internally.
Frequently Asked Questions
What does CAT code CID2-FMI4 mean?
CAT code CID2-FMI4 (Engine Coolant Temperature Sensor - Voltage Low / Short to Ground) is a moderate-severity fault on Caterpillar on-highway diesel engines. CID 2 FMI 4 is ECT signal voltage too low, almost always a shorted wire or bad sensor making the ECM think the engine is dangerously hot. Diagnose with CAT ET (Electronic Technician) software.
Can I drive my truck with CAT code CID2-FMI4?
Short distances may be acceptable, but running long with this fault risks progressive damage or forced derate. Address promptly.
How much does it cost to fix CAT CID2-FMI4?
Typical repair is $150-$400. Sensor / wiring fixes fall at the low end; injector, HEUI pump, turbo, or aftertreatment work falls at the high end. CAT dealer labor often requires CAT ET for calibration and parameter programming.
Sources
This page is built from documented references. Verify against your own service info before repair work.