U0631 code meaning
Lost Communication With Engine Coolant Temperature Sensor 5
Network system Generic code U06xx codes Check engine light
What U0631 means
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.
U0631 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.3 to 1.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0631
- Check engine light
- Fan running more often or longer
Common causes of U0631
- Most likelyWiring to sensor 5 damaged (open, short, chafing)
- PossibleConnector at sensor 5 corroded or not latched
- PossibleSensor 5 electronics failed or sending corrupt frames
- Less likelyShared supply or data line fault (often with U0630)
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Engine coolant temperature sensor 5Vehicle bus wiring
What each part of U0631 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 31 | The exact fault in that area. |
What to do next
- Note U0631 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about U0631
What does U0631 mean?
U0631 means "Lost Communication With Engine Coolant Temperature Sensor 5". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U0631?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will U0631 fail an emissions test?
Yes, it can. U0631 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is U0631 to fix?
Difficulty: medium. A typical repair takes 0.3 to 1.5 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will U0631 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.