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U0640 Lost Communication With EVAP Fuel Vapor Temperature Sensor Circuit

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

Many fuel tank pressure senders also measure the temperature of the vapor in the tank, and when such a sender reports digitally, typically over SENT, both values share one data stream. U0640 is stored when the engine controller no longer receives valid temperature data from this sensor. Despite the word circuit in the title, the fault concerns the digital link; an analog vapor temperature sensor reports wiring faults as P2024 to P2028. Audi documents U0640 together with U0641 for 2021–2023 A4 and A5 models with the 2.0 TFSI EA888: electrical interference on the vent system signal sets both codes sporadically, the cure is swapping two wires of a twisted bundle in the harness, and a new sender does not help. The MIL comes on, while the car drives normally.

Likely causes

Interference or chafing on the shared signal line of the tank pressure/temperature sender

Loose or corroded connector at the tank sender

Electronics of the sender failed (only if the code is stored permanently)

Engine controller software or input fault

Symptoms

Check engine light on
Often stored together with U0641
Evaporative system monitor does not complete

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€40–400

Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

evap vapor temp sensorvehicle bus wiring

References: static.nhtsa.gov, static.nhtsa.gov, melexis.com, law.cornell.edu.