Engine Coolant Temperature Sensor - Voltage Low / Short to Ground
What does CAT-CID2-FMI4 mean?
The CAT-CID2-FMI4 (Caterpillar) diesel fault code means: Engine Coolant Temperature Sensor - Voltage Low / Short to Ground. This is a moderate severity code.
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.