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DTC codes / U06xx · Network communication / U0640

U0640Lost Communication With EVAP Fuel Vapor Temperature Sensor Circuit

U0640 is a network communication code indicating that a control module on the vehicle's Controller Area Network (CAN) bus has lost contact with the EVAP fuel vapor temperature sensor circuit, typically because the sensor's smart module or its local wiring has stopped responding. The computer detected that messages or voltage signals expected from the fuel vapor temperature sensor ceased arriving within the required time window, so it flagged the missing communication path rather than a simple out-of-range temperature reading.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaNetwork communication

Symptoms you may notice

  • Illuminated check engine light or service vehicle soon message with no other obvious driveability complaint
  • Often no symptoms besides the warning light, because the EVAP system can default to a calculated temperature strategy
  • A pending or stored U0640 that returns immediately after clearing if the communication fault is hard
  • Other network-related codes or a U-code cluster if the fault lies on a shared CAN segment
  • Remote start or EVAP self-test functions may be inhibited on some vehicles
  • In rare cases, a slight fuel odor or EVAP monitor incomplete status if the sensor's data is used for purge control

Common causes, most likely first

  1. Open, chafed, or corroded wiring in the EVAP fuel vapor temperature sensor circuit or its connector
  2. Failed fuel vapor temperature sensor or its integrated smart module, causing no CAN response
  3. Blown fuse or poor ground shared by the sensor circuit or the module that hosts it
  4. CAN bus wiring fault such as an open data line, short to ground, or short to voltage on the segment serving the sensor
  5. Water intrusion or corrosion at the sensor connector or in the harness routing near the fuel tank
  6. Failed or intermittently dropping control module that gates communication to the sensor circuit
  7. Aftermarket accessory or poor previous repair disturbing the network wiring near the EVAP harness

How to diagnose and fix it

  1. Scan all modules and record every stored, pending, and permanent code to see whether U0640 is isolated or part of a network pattern
  2. Inspect the fuel vapor temperature sensor connector and harness for corrosion, water intrusion, chafing, and pushed terminals
  3. Check the fuse and ground circuits serving the sensor and its host module with a multimeter before condemning parts
  4. Measure CAN bus voltage and resistance at the sensor connector or the nearest accessible network node to confirm data-line integrity
  5. Perform a wiggle test on the harness while watching live sensor data or network communication status to expose intermittent faults
  6. Substitute a known-good sensor or module only after wiring and power/ground checks pass, then clear codes and verify the repair
  7. Recheck the EVAP monitor readiness and confirm U0640 does not return after a complete drive cycle

Why this code is often a wiring fault

  • U0640 is a communication loss code, not a temperature rationality code, so replacing the sensor before checking the connector and CAN wiring is the most common misdiagnosis trap.
  • A single failed sensor module on the network can set U0640 alongside unrelated U-codes, so always look at the full code list before deciding the fault is isolated to the EVAP sensor.
  • Water intrusion at the fuel tank area connector often causes this code intermittently, and the fault may only appear after rain or a car wash.
  • Some vehicles route the fuel vapor temperature sensor signal through a body or chassis module, so a fault in that module can set U0640 even when the sensor itself tests good.
  • Clearing U0640 without repairing the underlying communication fault will usually result in the code returning within one or two drive cycles.

Can I keep driving with U0640?

The vehicle is generally safe to drive because the EVAP system defaults to a backup strategy, but avoid ignoring the light for long since an unresolved network fault can spread to other modules. Do not clear the code repeatedly without diagnosis, and have the circuit inspected promptly if other warning lights or communication codes appear.

Tools that help

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Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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