Engine

P0078 — Exhaust Valve Control Solenoid Circuit Malfunction (Bank 1)

Severity: medium Drive with care, to a shop

What P0078 means

The engine control module (ECM) operates a solenoid that directs oil pressure to the variable valve timing mechanism on the Bank 1 exhaust valves. P0078 is stored when the electrical circuit of that solenoid does not behave as expected, such as an open, a short to ground or power, or out-of-range resistance. Bank 1 is the side of the engine that contains cylinder 1.

Symptoms

  • Check engine light on
  • Reduced power and sluggish acceleration
  • Rough or uneven idle
  • Worse fuel economy
  • Occasional hesitation or unstable rpm

Possible causes

  1. Failed exhaust valve control solenoid
  2. Damaged wiring: open circuit, short to ground or short to power
  3. Corroded or loose connector terminals
  4. Low, dirty or wrong-grade engine oil
  5. Clogged solenoid screen or oil passage
  6. Rarely, a failed driver inside the ECM

How to diagnose

  1. Scan all codes and freeze-frame data, clear the code and see whether it returns
  2. Check oil level and condition, and confirm the grade matches the manufacturer's spec
  3. Inspect the solenoid connector and harness for chafing, oil intrusion, corrosion and poor latching
  4. Measure the solenoid coil resistance and compare with the service manual value
  5. Test the wiring between the solenoid and ECM for opens and shorts to ground or power
  6. Check for power and the control signal at the connector with a test light or scope
  7. Remove the solenoid and inspect its screen for debris and check that it moves freely
  8. Suspect the ECM only after everything else is ruled out

Common fixes

  • Replace the exhaust valve control solenoid
  • Repair the wiring or replace a damaged connector
  • Change the oil and filter to the specified type and correct the level
  • Clean the solenoid screen and oil passages
  • Reprogram or replace the ECM if it is confirmed faulty

Questions and answers

What does P0078 mean?

It is an electrical circuit fault in the solenoid that controls exhaust valve timing on Bank 1. The ECM sees an open, a short or a wrong resistance in that circuit.

Can I drive with a P0078 code?

Briefly and carefully, yes, but the engine may lose power and run roughly. Get it fixed soon and check the oil level and quality in the meantime.

Can bad oil cause P0078?

Yes. Low, dirty or wrong-grade oil can hurt the variable timing system and clog the solenoid, even when the wiring itself is fine.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.