U0642: Lost Communication With EVAP System Pressure Sensor/Switch B

What it means

The vehicle bus reports loss of communication with smart EVAP system pressure sensor / switch B. Tank-pressure-based leak detection cannot run on this channel.

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

EVAP system pressure sensor or switch B reports pressure information used to check the fuel vapor system for leaks. The control module relies on network messages from the B channel for that pressure input. U0642 sets when the expected messages from B stop, so the B channel cannot provide its pressure data for EVAP monitoring.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. 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

1. Scan all modules for communication codes and review freeze-frame data to determine whether the fault is isolated to EVAP pressure sensor or switch B.
2. Check live data for the B channel pressure reading and compare with channel A, if fitted, to identify a missing or stale value.
3. Inspect sensor or switch B’s network connector and wiring for damage, corrosion, or loose terminals.
4. Check power and ground at the B sensor or its controller against the manufacturer’s specification.
5. Test the B channel network wiring for opens or shorts, then evaluate its reporting module if wiring and supplies pass.

Common fixes

• Reconnect or clean and secure the B channel network connector.
• Repair damaged or open network wiring to EVAP pressure sensor or switch B.
• Restore power or ground to the B sensor or controller.
• Replace the failed B channel reporting module if circuit checks confirm an internal fault.

Can I keep driving?

With no fuel odor or drivability symptoms, normal short-term driving is generally possible, but EVAP leak detection may be impaired and emissions readiness may not complete. Arrange diagnosis before an emissions inspection, and address any fuel smell promptly.