Code
U0641
Generic
U — Network/User
Lost Communication With EVAP System Pressure Sensor/Switch Circuit A
AI status
Completed
Completed
100%
Causes
- Open, shorted or damaged wiring between sensor and control module
- Corroded, loose or contaminated connector at the EVAP pressure sensor
- Failed EVAP pressure sensor or sensor switch
- Loss of sensor reference power or ground (blown fuse, poor ground)
- Intermittent connection (vibration, heat-related)
- Faulty PCM/EVAP control module or internal module communications fault
Symptoms
- Check Engine Light (MIL) illuminated
- Stored U0641 with diagnostic trouble codes related to EVAP
- EVAP system self-test not completed or not available
- Possible trouble codes for related modules (loss of comm messages)
- No obvious driveability symptoms in many cases
What to check
- Read and record all DTCs and freeze-frame data with a scan tool; note related codes
- Visually inspect EVAP pressure sensor, connector and wiring for corrosion, damage or water ingress
- Check fuses and power supply circuits for the EVAP sensor/EVAP module
- Backprobe sensor connector: verify reference voltage (often ~5 V) and ground presence
- Measure sensor signal voltage while key ON and during engine run, compare to expected range
- Wiggle harness and connectors while observing live data for intermittent faults
Signal parameters
- Reference voltage: typically ~5.0 V (check vehicle-specific spec)
- Signal output: typically 0.5–4.5 V proportional to measured EVAP pressure (varies with model)
- Ground:
- If sensor is a switch: open/closed continuity on switch circuit when actuated
- Digital sensors: valid message on vehicle bus (CAN/LIN) containing EVAP pressure data
Diagnostic algorithm
- Retrieve and save all diagnostic codes and freeze-frame information. Note when U0641 first set.
- Inspect the EVAP pressure sensor harness and connector for damage, corrosion, loose pins or water.
- Verify power and ground at the sensor connector with key ON. If missing, trace/repair fuse, power feed or ground.
- Backprobe the signal wire and measure voltage while operating system or applying known pressure; compare to spec.
- Wiggle the harness and connectors while monitoring live data to locate intermittent faults.
- If the vehicle uses a networked sensor, check the vehicle bus (CAN/LIN) for activity and other module comms; scan for related U‑codes.
- Repair damaged wiring, clean/replace corroded connectors, and secure grounds. Re-test after each repair action.
- If wiring and supply checks are good and sensor signal is out of range or absent, replace the EVAP pressure sensor/switch.
- After repair or replacement, clear codes, perform readiness/EVAP tests and road test to confirm the issue does not return.
- If fault persists after sensor replacement and harness repair, consider module-level diagnostics or software update from manufacturer.
Likely causes
- Damaged/loose connector at the EVAP pressure sensor
- Broken or chafed wiring in harness to sensor
- Failed sensor (most likely if wiring and power/ground are good)
- Blown fuse or poor ground to EVAP control circuit
- Intermittent wiring fault revealed under movement or temperature change
- Module communication fault or rare ECU internal failure
Fault status
Status
Lost communication with EVAP system pressure sensor/switch circuit A — no valid signal/data received from the sensor.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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