P0078

What it means

P0078 means the engine control unit has detected a fault in the electrical circuit of the exhaust valve control solenoid on Bank 1. This solenoid is part of a variable valve timing or valve lift system on the exhaust side. The control unit compares the commanded state of the solenoid driver with the voltage or current it sees and sets the code when they do not agree, for example an open circuit, short circuit or out-of-range reading.

Symptoms

• Engine warning light on
• Rough or unstable idle
• Reduced power or poor acceleration
• Poor fuel economy
• Engine running in a limited performance mode on some models
• Hesitation or uneven running at certain engine speeds

Possible causes

• Damaged, chafed or corroded wiring between the control unit and the exhaust valve control solenoidlikely
• Poor connection at the solenoid plug or control unit connector (corrosion, bent or loose pins, oil contamination)likely
• Faulty exhaust valve control solenoid with open or shorted coillikely
• Low or poor quality engine oil, or a blocked solenoid screen, affecting solenoid operation where the system is oil-pressure drivenpossible
• Low battery voltage or a poor earth or supply affecting the solenoid circuitpossible
• Fault in the control unit output driver or an outdated calibration known for this code on certain modelsrare

How to diagnose

1. Read all stored codes and freeze frame data to see engine speed, load, temperature and battery voltage when the code set, and note any other valve timing or related codes.
2. Check engine oil level and condition, as some variable valve systems depend on clean oil at the correct level.
3. Visually inspect the solenoid, its connector and the wiring on Bank 1 for oil soaking, chafing, heat damage, corrosion or loose pins.
4. Check battery condition and charging voltage, and confirm the solenoid supply and earth are sound.
5. Measure the solenoid coil resistance and compare it with the manufacturer specification for the model and engine.
6. Check the wiring for continuity, short to earth and short to supply between the solenoid and control unit, with the connectors disconnected.
7. Command the solenoid with a suitable diagnostic function if available and confirm it operates and the control unit sees the response.
8. Check for applicable manufacturer service bulletins or calibration updates for the specific engine before condemning the control unit.

How to fix

• Repair or replace damaged wiring and fix any chafing or short circuits.
• Clean or replace the affected connector terminals, and remove oil contamination from the harness.
• Replace the exhaust valve control solenoid if its resistance or operation is out of specification.
• Correct the oil level, change the oil and filter if it is degraded, and clean or replace the solenoid screen where fitted.
• Repair battery, supply or earth faults, then clear the code and road test to confirm it does not return.
• Update the control unit calibration only where the manufacturer lists it as a fix, and replace the unit only after all other causes are ruled out.

Can I keep driving?

Nearby codes

• P0077GenericIntake Valve Control Solenoid Circuit High Bank 1
• P0079GenericExhaust Valve Control Solenoid Circuit Low Bank 1