U0638 code meaning
Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor A
Network system Generic code U06xx codes Check engine light
What U0638 means
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.
U0638 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyHard
- Typical labor0.5 to 2.5 hours
- Clears after repairWhen cleared with a scanner
Symptoms of U0638
- 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
Common causes of U0638
- Most 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
- Less likelyFault or outdated software in the controller that polls the sensor
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Hv electronics coolant temp sensor aVehicle bus wiring
What each part of U0638 means
| U | Network system: communication between the control modules (CAN bus). |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 38 | The exact fault in that area. |
What to do next
- Note U0638 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 U0638
What does U0638 mean?
U0638 means "Lost Communication With Hybrid/EV Electronics Coolant Temperature Sensor A". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with U0638?
Drive gently and only to a workshop. With U0638 the car can go into limp mode (reduced power) to protect itself.
Will U0638 fail an emissions test?
Yes, it can. U0638 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 U0638 to fix?
Difficulty: hard. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will U0638 clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.