Engine Coolant Temperature Sensor - Voltage High / Open Circuit
The CAT-CID2-FMI3 (Caterpillar) diesel fault code means: Engine Coolant Temperature Sensor - Voltage High / Open Circuit. This is a moderate severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- intermediate
- Estimated cost
- $150-$450
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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
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.
Mechanics Guide Step-by-step diagnosis
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.
Sources 1
This page is built from documented references. Verify against your own service info before repair work.
- SAE J1939-73Application Layer: Diagnostics (defines the SPN/FMI fault-identifier framework)
Commonly Affected Vehicles
- Freightliner Classic XL with CAT C15
- Peterbilt 335 with CAT C7
- Peterbilt 386 with CAT C15
- Kenworth T800 with CAT C15
- Western Star 5900 with CAT C15
- Peterbilt 389 with CAT C15
- Peterbilt 387 with CAT C15
- Kenworth T470 with CAT C9
- Kenworth T800 with CAT C13
- Peterbilt 386 with CAT C13
- Thomas / Bluebird school bus with CAT C7
- Peterbilt 387 with CAT C13
- Peterbilt 340 with CAT C9
- Peterbilt 379 with CAT C15
- Kenworth T370 with CAT C7
- Kenworth T660 with CAT C13
- International 9400i with CAT C15
- Kenworth W900 with CAT C15
- Freightliner Columbia with CAT C13
- International 9200i with CAT C13
- International 4300 with CAT C7
- Western Star 4900 with CAT C15
- Freightliner M2 106 with CAT C7
- IC Bus shuttle with CAT C7
- International 9200i with CAT C15
- Kenworth T2000 with CAT C15
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