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P codepowertrainMedium severity

P0657Actuator Supply Voltage "A" Circuit/Open

The switched supply voltage the PCM feeds to its "A" group of actuators is missing or open when it should be present.

Definition

The PCM supplies a switched voltage feed to a group of actuators — commonly the throttle actuator motor, transmission solenoids, or emissions solenoids — through internal high-side drivers, and it monitors that supply. P0657 sets when the actuator supply voltage "A" circuit is open or absent after being commanded on. Because the affected actuators lose power together, this code frequently appears with reduced engine power or limp mode, harsh or default transmission operation, and companion actuator circuit codes; the MIL illuminates. On several GM applications this code accompanies a "Reduced Engine Power" message.

Common causes

  • Blown fuse or failed relay feeding the PCM's actuator supply circuit
  • Open or corroded wiring in the supply feed between the fuse block and PCM
  • Poor PCM power or ground connections causing the internal driver to drop out
  • Chafed harness with an intermittent open at connectors or splice packs
  • Water intrusion or corrosion in the underhood fuse block
  • Failed internal supply driver in the PCM

Common misdiagnoses

  • Throttle body replaced for the reduced-power symptom when the supply feed is dead
  • Transmission solenoids or valve body replaced for companion solenoid codes
  • Battery or alternator replaced for a localized supply fault
  • PCM replaced before checking fuses, relays, and grounds

Troubleshooting steps

  1. 1.Pull all codes and note the pattern

    Record companion codes; multiple actuator/solenoid circuit codes setting simultaneously with P0657 confirm a shared supply failure rather than individual component faults.

  2. 2.Check fuses and relays

    Test the PCM/ignition and actuator supply fuses under load and swap or test the feed relay; a fuse that blows again indicates a downstream short to find.

  3. 3.Verify PCM powers and grounds

    Voltage-drop test every PCM power and ground circuit (target under 0.1 V drop) since a bad ground can mimic an internal supply failure.

  4. 4.Measure the actuator supply at the PCM

    Backprobe the actuator supply output pin key-on; battery voltage should appear when commanded — absent voltage with good inputs points to the PCM driver, present voltage points to a downstream open.

  5. 5.Wiggle-test the harness

    Flex the harness between the fuse block, PCM, and affected actuators while watching for the supply to drop, focusing on connectors and areas near heat or moving parts.

  6. 6.Replace or reprogram the PCM only after circuits pass

    Condemn the PCM only when supply feeds, grounds, and the downstream circuit have all tested good and the internal driver still will not deliver voltage.

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