Code P0078 indicates a malfunction in the electrical circuit of the exhaust valve control solenoid for bank 1. This solenoid manages exhaust valve timing, and the code sets when the powertrain control module (PCM) detects an open, short, or other circuit fault.
The solenoid itself may be stuck, have an internal open or short circuit, or be mechanically worn, preventing proper operation.
Wiring between the PCM and solenoid may be shorted to ground or voltage, open, or have high resistance due to corrosion, chafing, or breaks.
Loose, corroded, or pushed-out terminals in the solenoid connector can cause intermittent or high-resistance circuit faults.
A blown fuse or defective relay in the solenoid power supply circuit can interrupt voltage to the solenoid.
The PCM driver for the solenoid circuit may be damaged, but this is uncommon and should only be considered after all other causes are ruled out.
It regulates oil flow to the exhaust camshaft phaser, adjusting valve timing for optimal performance, fuel economy, and emissions.
Short-distance driving may be possible, but prolonged operation can cause poor fuel economy, reduced power, and potential engine damage. It is best to diagnose and repair promptly.
No, PCM failure is very rare. The solenoid itself, wiring issues, or connector problems are far more common. The PCM should only be suspected after all other possibilities are thoroughly tested.
Diagnose the circuit first: check the solenoid resistance, wiring continuity, and power supply. Replace the solenoid if faulty, repair any damaged wiring, and clear the code. Avoid replacing parts without proper testing.