Intake Air Temperature Sensor - Voltage Above Normal (Open/Short to High)
What does CAT-CID172-FMI3 mean?
The CAT-CID172-FMI3 (Caterpillar) diesel fault code means: Intake Air Temperature Sensor - Voltage Above Normal (Open/Short to High). This is a moderate severity code.
Common Symptoms
- Check Engine Light
- Sensor reads pegged high or low on CAT ET
- Associated systems may derate or default
Probable Causes (Ranked by Likelihood)
- Open circuit in signal wire or unplugged connector Very Likely
- Sensor internally failed open Likely
- Corroded or backed-out connector pin Likely
- Signal wire shorted to sensor supply Possible
Step-by-Step Diagnostic Procedure
Connect CAT ET and confirm the CID and FMI is active. Note the reported sensor reading.
Disconnect the sensor connector and verify reference voltage at the supply pin key-on.
Jumper signal to return at the connector; if the value swings to the opposite extreme on ET, the sensor is bad.
If no swing, ohm signal wire from connector back to ECM pin; repair open or replace sensor as indicated.
Frequently Asked Questions
What does CAT code CID172-FMI3 mean?
CAT code CID172-FMI3 (Intake Air Temperature Sensor - Voltage Above Normal (Open/Short to High)) is a moderate-severity fault on Caterpillar on-highway diesel engines. CID 172 is the Intake Air Temperature Sensor, measuring intake air temperature. FMI 3 (Voltage Above Normal (Open/Short to High)) indicates the specific fault mode. Use CAT ET to confirm before replacing parts. Diagnose with CAT ET (Electronic Technician) software.
Can I drive my truck with CAT code CID172-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 CID172-FMI3?
Typical repair is $150-$500. 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.