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U0642 Lost Communication With EVAP System Pressure Sensor/Switch Circuit B

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modeNo

A second EVAP pressure sensor, B, appears where one reading is not enough, for instance when tank and canister are separated by an isolation valve and each side needs its own value, as in sealed tanks of plug-in hybrids that see higher pressures. Which location is called B is up to the manufacturer. U0642 is the lost-communication code for this sensor when it reports digitally; its analog counterpart logs P24D5 to P24D9. No manufacturer documents U0642 publicly. Because CARB 1968.2 (e)(15.2.1)(B)(i) demands a separate code for a digital input that stops talking, the code exists independently of U0641. While B is silent, the controller cannot compare both sides or check the isolation valve, so the leak test is suspended and a MIL is to be expected.

Likely causes

Signal, supply or ground line to sensor B open or chafed

Corroded or unplugged connector at sensor B

Sensor B electronics failed

Receiving controller input or software fault

Symptoms

MIL on
EVAP monitor incomplete
On sealed tanks possibly a delayed fuel door release

Repair estimate

DifficultyMediumWorkshop job

Parts and labour€40–450

Time0.5–3 h

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

Parts involved

evap pressure sensor bvehicle bus wiring

References: law.cornell.edu, melexis.com, en.wikipedia.org.