U0638Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor A
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- The coolant loop that serves the inverter and other high-voltage electronics needs its own temperature reading so the controller can size pump speed and decide when to derate. When sensor A on that loop is a bus node that sends its value digitally, for instance over LIN, the receiving controller notices when frames stop coming, and U0638 records that silence. No manufacturer documents this code publicly. CARB 13 CCR 1968.2 (e)(15.2.3)(B)(ii) requires every electronic input that serves inverter cooling to be checked like other input components, so a MIL is expected on hybrids. An analog sensor of the same position reports wiring faults through P0CEE to P0CF2 instead. Without a reading the system usually assumes a warm loop and limits electric power.
- What you may notice
- Hybrid system warning, on hybrids usually the MIL as well
- Reduced electric drive power, especially under load or in hot weather
- Electronics coolant pump running continuously at high speed
- Likely causes
- LikelyData, supply or ground line to sensor A open, shorted or chafed
- PossibleCorroded or loose connector at the electronics coolant line
- PossibleInternal electronics of sensor A failed
- RareFault or outdated software in the controller that polls the sensor
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €80 – €600
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.