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P codepowertrainMedium severity

P0078Exhaust Valve Control Solenoid Circuit (Bank 1)

The computer has detected an electrical fault in the Bank 1 exhaust valve control solenoid circuit.

Definition

The PCM drives the Bank 1 exhaust valve control solenoid to manage variable exhaust valve timing or lift and monitors that circuit for electrical faults. P0078 sets on a general malfunction — an open, intermittent, or implausible electrical condition — in the circuit. The MIL illuminates and exhaust valve control defaults to base position, which can cause reduced power, poor fuel economy, increased emissions, and on some engines a rattling cold-start or rough idle.

Common causes

  • Open or intermittent wiring between the PCM and the Bank 1 exhaust valve control solenoid
  • Corroded or oil-contaminated connector terminals
  • Failed solenoid winding out of resistance specification
  • Chafed harness shorting to ground or power
  • Lost fuse/supply feed to the solenoid
  • Faulty PCM output driver (rare)

Common misdiagnoses

  • Replacing the solenoid when the wiring or connector is at fault
  • Replacing the exhaust cam phaser for a purely electrical circuit code
  • Replacing the camshaft position sensor, which is a separate circuit
  • Condemning the PCM without proving the circuit at both ends

Troubleshooting steps

  1. 1.Inspect the connector and harness

    Check the Bank 1 exhaust solenoid connector for corrosion, oil intrusion, and loose terminals, then trace the harness for visible damage.

  2. 2.Measure solenoid resistance

    Compare the winding to the published spec; readings open, near zero, or well out of range condemn the solenoid.

  3. 3.Verify power and control circuits

    Confirm key-on supply voltage at the connector and check the PCM control wire for continuity and freedom from shorts.

  4. 4.Command the solenoid

    Use a bidirectional scan tool to energize the solenoid and confirm an audible click and a change in exhaust cam position data.

  5. 5.Wiggle-test for intermittents

    Flex the harness at connectors and clip points while monitoring the circuit to expose broken strands that only open under vibration.

  6. 6.Clear and road test

    After repair, clear the code and confirm commanded versus actual exhaust valve position track together through a full warm-up drive.

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