Exhaust Valve Control Solenoid Circuit Bank 1


P0078 Indicates that the PCM has detected an incorrect voltage reading or degree of resistance from the VVT circuit that controls exhaust valve timing for engine bank 1.

Code Set Parameters

Any variation of greater than 10-percent from the manufacturer’s reference voltage will result in a stored code and an illuminated service engine soon lamp.


Symptoms could include a rough idle, choppy acceleration, poor engine performance, and reduced fuel efficiency accompanied by an illuminated malfunction indicator lamp.

Common Causes

Probable causes include low engine oil level/pressure, a faulty control solenoid, faulty PCM, corroded, broken, or loose electrical wiring or connectors in the VVT electrical circuit.

Common Misdiagnosis

Components are frequently replaced in error when a low oil level or oil pressure condition could be the culprit. The VVT/VCT system utilizes electrical solenoids and engine oil pressure to help activate the valves in question.


  • Begin your diagnosis by visually inspecting all wiring and connectors
  • Look for shorted or burned wiring and replace circuitry and connectors as required
  • If the system wiring, connectors, and components appear to be in normal working order, connect the scanner to the diagnostic connector and record all stored trouble codes and freeze frame data
  • This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
  • After the codes are cleared, operate the vehicle to see if the code returns
  • If the code fails to immediately return, you may have an intermittent condition
  • Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made. Inspect electrical wiring and connectors for inconsistencies, including shorted or broken wiring (especially where corrosive fluids are present) and corroded connector faces
  • Verify that all timing components are properly aligned and that engine oil level and engine oil pressure are within acceptable specifications
  • If these early checks are acceptable, then test system components and voltage/resistance using manufacturer’s reference voltage and resistance figures