U0640

Code

Generic U — Network/User

Lost Communication With EVAP Fuel Vapor Temperature Sensor Circuit

Brand: Generic

Type: U — Network/User

Views: UK: 21 EN: 35 RU: 28

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Page language: EN

Causes

Open, short, chafed, or corroded wiring between the EVAP temp sensor and the control module
Poor or corroded connector pins at sensor, module, or inline connectors
Failed EVAP fuel vapor temperature sensor
Failed EVAP control module or related ECU
Communication bus fault (CAN, LIN or proprietary network) or lost reference power/ground
Software/firmware issue or missing module address on the network

Symptoms

MIL (Check Engine Light) illuminated and stored U0640 trouble code
Possible multiple EVAP-related DTCs (evap leaks, purge/vent faults)
No fuel vapor temperature data available on a scan tool
Reduced emissions control function or failed emissions test
Occasional poor driveability only if EVAP system logic is affected

What to check

Retrieve all stored codes and freeze-frame data with a scan tool; note related EVAP or network codes
Check for current network/module presence using a capable scan tool (verify if the EVAP sensor or module appears)
Visually inspect wiring and connectors from the EVAP temp sensor to the EVAP module/ECU for damage, corrosion, or water intrusion
Verify sensor connector is fully seated and pins are not bent, pushed out, or corroded
Backprobe sensor connector to confirm reference voltage, ground continuity, and signal while key ON and during operation
Check for continuity and short-to-ground/short-to-battery on the sensor power, ground, and signal wires

Signal parameters

Sensor supply/reference voltage typically ~5 V (verify with service manual for vehicle-specific value)
Sensor ground near 0 V (ground continuity to chassis battery negative)
Analog temperature output typically varies with temperature — often ~0.5–4.5 V (varies by design; consult spec)
If on CAN bus: CAN_H and CAN_L idle near 2.5 V each (differential ~0 V idle); active frames show expected differential swings — check with scope
If on LIN bus: LIN idle is near battery voltage; active frames are low pulses (vehicle-specific — consult service manual)

Diagnostic algorithm

Use a scan tool to record U0640 and any related codes, note freeze frame and whether the EVAP temp sensor parameter is visible or missing.
Attempt to communicate with the EVAP module/sensor via manufacturer scan tool functions; check if the module responds on the network.
Perform a careful visual inspection of the EVAP sensor, harness, and connectors for damage, corrosion, or contamination. Repair any obvious harness issues.
Backprobe the sensor connector with key ON: verify reference voltage (typically ~5 V), verify ground continuity, and observe signal voltage. Compare to expected ranges in service data.
Check wiring continuity between the sensor connector and the EVAP module/ECU; check for shorts to battery and ground.
If the sensor uses a serial bus (CAN/LIN), monitor the bus with a scan tool or oscilloscope for proper idle voltages and activity. Look for missing or corrupt frames.
Wiggle test connectors and harness while monitoring data to find intermittent faults (do this safely and with the vehicle stationary).
If wiring and connectors are good but no communication, substitute a known-good sensor (if available) or bench-test the sensor per manufacturer procedure.
If the sensor and wiring check good, suspect the EVAP control module or bus master device; verify other modules communicate on the same bus. Consider reflashing or replacing module per service instructions.
After repairs, clear codes and perform a drive cycle to verify the fault does not return; confirm proper EVAP temp data is present on the scan tool.

Likely causes

Connector corrosion or loose connector at the EVAP temp sensor
Open or short in the sensor signal or power/ground wiring
Failed EVAP fuel vapor temperature sensor
Bus communication fault (CAN/LIN) or missing module on network

Fault status

⚠️ Status

Lost Communication With EVAP Fuel Vapor Temperature Sensor Circuit — the EVAP temperature sensor is not providing data to the control module or is not present on the vehicle network. Check wiring, connectors, sensor, and CAN/LIN bus.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1.0-3.0 hours