Engine Coolant Temperature Sensor - Voltage High / Open Circuit
What does CAT-CID2-FMI3 mean?
The CAT-CID2-FMI3 (Caterpillar) diesel fault code means: Engine Coolant Temperature Sensor - Voltage High / Open Circuit. This is a moderate severity code.
Common Symptoms
- Check Engine Light or amber warning lamp
- Coolant temp reads -40°F or max on CAT ET
- Fan may run constantly or not at all
- Possible engine derate if temp reads excessive
- Cold-start enrichment may be wrong
Probable Causes (Ranked by Likelihood)
- Open circuit in ECT sensor signal or return wire Very Likely
- ECT sensor internally failed (open) Likely
- Connector pin push-back or corrosion at sensor Likely
- Harness chafed against engine block or bracket Possible
- ECM signal circuit internal failure Less Likely
Step-by-Step Diagnostic Procedure
Connect CAT ET and verify CID 2 FMI 3 is active; note ECT reading (likely -40°F).
Disconnect ECT sensor connector and check for 5V at signal pin key-on.
Jumper signal to return at connector — ECT should jump to ~260°F on ET. If it does, sensor is bad.
If no jump, measure signal wire continuity from connector to ECM pin, and return to ECM ground.
Inspect pin fit and backshell corrosion at the sensor. Replace sensor or repair harness as found.
Frequently Asked Questions
What does CAT code CID2-FMI3 mean?
CAT code CID2-FMI3 (Engine Coolant Temperature Sensor - Voltage High / Open Circuit) is a moderate-severity fault on Caterpillar on-highway diesel engines. CID 2 is the engine coolant temperature sensor. FMI 3 means the ECM sees the signal voltage above normal — almost always an open circuit, unplugged connector, or internally open sensor. Diagnose with CAT ET (Electronic Technician) software.
Can I drive my truck with CAT code CID2-FMI3?
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-FMI3?
Typical repair is $150-$450. 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.