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U0641 Lost Communication With EVAP System Pressure Sensor/Switch Circuit A

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

Pressure sensor A is the main input for the evaporative leak test that CARB 1968.2 (e)(4) requires: with the system sealed, the controller follows how pressure in tank and canister rises or falls. Newer designs often use a relative pressure sensor with SENT output; Melexis' MLX90821, for example, spans about −0.06 to 0.26 bar in its SENT version and can add a media temperature. U0641 means these digital messages are missing or unusable, which (e)(15.2.1)(B)(i) treats as its own fault; an analog sensor would log P0450 to P0454. Audi's 2.0 TFSI bulletin for 2021–2023 A4/A5 separates sporadic entries, caused by interference and fixed in the harness, from permanent ones that need normal diagnosis. The leak test stays blocked and the MIL is on.

Likely causes

Electrical interference or damage on the sensor signal line

Connector at the tank or canister sensor loose, wet or corroded

Pressure sensor A itself no longer sends

Sensor supply or ground missing

Symptoms

MIL on
EVAP leak monitor not ready, possible inspection failure
No noticeable change in driving

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€40–450

Time0.5–2.5 h

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

Parts involved

evap pressure sensor avehicle bus wiring

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