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U0657 Lost Communication With Turbocharger/Supercharger Bypass Valve A Position Sensor

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

The bypass valve meant here sits on the intake side: on turbocharged petrol engines it lets compressed air recirculate when the throttle shuts, and on supercharged engines it routes air around the blower at light load. When the valve is electrically driven and reports its position digitally, U0657 marks that the position data from valve A has stopped arriving. No manufacturer publishes a description of U0657, so the exact valve and its protocol come from the vehicle's own service information. Analog circuit faults of the same sensor are P2BBB to P2BBD and an adaptation problem is P2BBE; valve B has U066B. Without position feedback the controller cannot tell whether surplus air is being released, so it may restrict boost to protect the compressor.

Likely causes

Wiring to the bypass valve open, shorted or chafed
Loose or corroded valve connector
Position sensor in the valve failed
Engine controller input fault

Symptoms

  • MIL on
  • Less boost or hesitant throttle response
  • Unusual whoosh or flutter when lifting off

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–600
Time0.5–2.5 h

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

Parts involved

turbo bypass valve a position sensorvehicle bus wiring

Related codes

U066BP2BBBP2BBCP2BBDP2BBEU0656

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