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?
- Short distances only
- 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
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.
Mechanics Guide Step-by-step diagnosis
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.
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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