U0631Lost Communication With Engine Coolant Temperature Sensor 5
- Generic OBD-II code · Network
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- Sensor 5 is the highest-numbered engine coolant temperature input in the J2012 list and is found only on engines that monitor many coolant zones, for example block, head, radiator outlet and separate loops. U0631 records that the controller has stopped receiving digital messages from that sensor. No manufacturer documents it publicly. Because sensor 4 and sensor 5 usually sit on different branches, U0631 on its own points to its own connector, wire or sensor electronics, while both together suggest a shared data line or supply. CARB folds sensors that watch distinct coolant loops into the cooling system monitor, so the MIL is plausible. LIN messages carry a checksum, and a sensor that answers with bad frames counts as missing too. Expect a substitute value and cautious fan control.
- What you may notice
- Check engine light
- Fan running more often or longer
- Likely causes
- LikelyWiring to sensor 5 damaged (open, short, chafing)
- PossibleConnector at sensor 5 corroded or not latched
- PossibleSensor 5 electronics failed or sending corrupt frames
- RareShared supply or data line fault (often with U0630)
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €30 – €300
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.