P2AC9Charge Air Cooler Coolant Temperature Sensor B Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P2AC9 reports that the signal of charge air cooler coolant temperature sensor B has dropped below its permitted window. These sensors are NTC thermistors, so a low signal stands for an implausibly hot coolant and usually means the signal wire touches ground or the element is internally shorted. Public limits exist only for sensor A: Ford's 2023 diesels flag P00E0 below 0.162 V within 5 s, while GM's 2025 diesel table reads resistance and fails under 70 ohms in 30 of 38 samples as a Type B code. A B-sensor check from the same maker is likely built the same way, but no document states it. With a false hot value the control may run the loop pump constantly or cut boost. Measure resistance at a known temperature and inspect the harness for chafing.
- What you may notice
- Check engine light on
- Live data shows an extremely high coolant temperature for sensor B
- Coolant pump of the loop runs constantly or boost is reduced
- Likely causes
- LikelySignal wire of sensor B shorted to ground
- PossibleSensor B internally shorted
- RareCoolant in the connector bridging the terminals
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Easy · Often possible to do yourself.Typical cost: €60 – €300
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.