OBD-II code U0642 indicates the powertrain control module (PCM) has lost communication on the data network with the evaporative emission (EVAP) system pressure sensor/switch Circuit B. This is a network communication fault, not a fuel-system performance fault, meaning the sensor is either not responding on the bus or its power/ground/network wiring is disrupted. The code commonly appears alongside other EVAP-related codes and is most frequently caused by wiring or connector problems rather than a failed sensor.
Chafed, broken, water-intruded, or corroded harness and pins on the sensor connector or CAN/power/ground wiring are the most frequent cause of a 'Lost Communication' code.
The sensor itself can fail internally so that it no longer responds on the network or no longer produces a valid signal to the module reading it.
A blown fuse, broken ground strap, or shared ground point failure will prevent the sensor from powering up and communicating, even if the wiring to it is intact.
A short, open, or high resistance on the CAN high/low circuits servicing the EVAP sensor can keep it offline and generate a lost-communication fault.
Water intrusion, a cracked hose, or a stuck/damaged sensor port can produce a no-response condition on some systems, especially after a fuel fill or cold soak.
An internal failure of the module that interfaces with the sensor is the least common cause and should be considered only after every wiring, connector, power, ground, and sensor test has been ruled out.
Usually not. U0642 is a lost-communication code, and the large majority of cases are caused by wiring, connector, power/ground, or sensor faults. The PCM is the least likely cause and should only be considered after all of the above have been ruled out with proper testing.
In most cases, yes. The vehicle typically still runs and drives normally, but the EVAP emissions monitor will be incomplete, which can cause a failed emissions or smog inspection until the fault is repaired.
Not directly. A loose or leaking gas cap usually sets a different code (commonly P0455 or P0442). U0642 points specifically at communication with the EVAP pressure sensor Circuit B rather than a simple vapor leak, although a damaged seal or hose near that sensor can contribute.
No. Clearing or erasing the code only removes the stored information temporarily; the code will return as soon as the same condition is detected on the next drive cycle, which is why proper diagnosis of the wiring, power, ground, and sensor is required.