U0640 indicates the Powertrain Control Module (PCM) has lost communication with the Evaporative Emission (EVAP) system fuel vapor temperature sensor. This is a network communication fault, meaning the sensor or its circuit is not responding on the Controller Area Network (CAN) bus. The EVAP system manages fuel vapor recovery to prevent fuel vapors from escaping into the atmosphere.
The sensor itself has failed and is no longer responding to PCM queries. This is the most common cause of U0640.
Open circuits, short circuits, or damaged wiring between the PCM and the sensor, often due to age, heat, or physical damage.
Moisture intrusion, corrosion, or poor terminal contact at the sensor connector or PCM connector.
A failed or high-resistance ground circuit for the sensor, preventing proper communication.
If the sensor requires switched power, a blown fuse or failed relay can cause loss of communication.
Extremely rare as a primary cause; the PCM itself may have failed only if all other causes are definitively ruled out.
Yes, the vehicle will typically drive normally. However, the EVAP system may not function properly, which could lead to increased fuel vapor emissions and potential failure of emissions testing.
Yes, in most cases the sensor itself is the culprit and replacement will resolve the code. However, proper diagnosis should first confirm the sensor, wiring, and connections are at fault before replacement.
Some shops may recommend PCM replacement because it is a quick solution that avoids detailed circuit diagnosis. However, a failed PCM is rarely the cause of U0640. The sensor, wiring, or connectors are the actual problem in the vast majority of cases. Always insist on proper diagnostic steps to verify before spending on a control module replacement.