What it means
The vehicle bus reports loss of communication with smart EVAP fuel vapor temperature sensor. Tank-vapor temperature compensation for the EVAP system is unavailable.
At a glance
- Usually lights the warning light of the system that lost communication
- Affects emissions; the car can fail an emissions test
How it works
The EVAP fuel vapor temperature sensor sends vapor temperature information over the vehicle network for evaporative emissions monitoring. The control module expects periodic messages from this smart sensor. U0640 sets when those messages stop arriving, so EVAP monitoring may lose the vapor temperature input and related emissions checks may be affected.
Common causes
Most likely first.
- Bus wiring open or shorted
- Module without power or ground
- Module failed
Symptoms
- Multiple warning lights may illuminate
- Affected modules may go into limp mode or shut down
- Specific functions inoperative depending on module
- Diagnostic tools may show communication faults
How to diagnose it
- Scan all modules for network codes and review freeze-frame data to see whether other devices lost communication at the same time.
- Check live data for EVAP fuel vapor temperature to confirm whether its reading is absent or no longer updating.
- Inspect the sensor’s network connector and wiring for damage, corrosion, loose terminals, or exposure near the fuel tank.
- Check power and ground at the sensor or its controller against the manufacturer’s specification.
- Test the network circuit for opens or shorts and assess the sensor’s reporting module if the wiring and supplies are sound.
Common fixes
- Reconnect or repair a loose or damaged EVAP sensor network connector.
- Repair an open or short in the sensor’s network wiring.
- Restore power or ground to the sensor or its controller.
- Replace the failed sensor reporting module after confirming its circuit is intact.
Can I keep driving?
The code usually affects EVAP monitoring rather than immediate drivability, so normal short-term driving is generally possible if there are no fuel or other system warnings. The emissions check may not complete, and a fuel odor or warning affecting vehicle operation warrants prompt attention.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.