P0078
PowertrainExhaust Valve Control Solenoid Circuit (Bank 1)
The ECU sends a control signal to the exhaust-side valve timing solenoid on bank 1 and watches the electrical feedback from that circuit. When the voltage or resistance it sees doesn't match what it expects, it logs P0078 and assumes the solenoid or its wiring has a fault. For you as the driver, that usually means the engine can't adjust its exhaust valve timing properly, so it may feel flat in places and the engine light will be on. Sometimes the solenoid is fine and it's just a tired connector or low oil starving the system.
ⓘ Information only. This page provides general educational information about fault code P0078. We do not assess the urgency or safety implications of any specific fault. That requires in-person diagnosis by a qualified mechanic. Full terms.
Recommended next steps
Whether a fault is urgent, drivable, or routine depends entirely on the cause on a specific vehicle, and that can only be determined by a qualified mechanic with diagnostic equipment. If a warning light is illuminated, the most reliable next step is professional diagnosis.
What does P0078 mean?
P0078 is a Powertrain (engine, transmission, fuel system) fault code. It indicates: Exhaust Valve Control Solenoid Circuit (Bank 1).
This is a standardised OBD-II code. The technical definition is the same regardless of the make or model of vehicle, although specific causes and symptoms can vary between vehicles.
Symptoms commonly associated with this code
Symptoms that drivers often report alongside this code. Not all may apply to every case:
- • Engine management light on, sometimes with no other obvious change to how the car drives
- • Noticeably flat acceleration under moderate load, like the engine is holding itself back
- • Lumpy idle that you can feel through the seat or steering wheel
- • The odd stall when you lift off the throttle coming to a junction
- • Fuel economy creeping up, more obvious on a regular commute than a one-off run
- • Some cars drop into limp mode and cap the revs until the fault clears
Possible causes
Causes commonly associated with P0078, listed in approximate order of typical investigation. The actual cause on a specific vehicle can only be confirmed by professional diagnosis.
- 1. Damaged or oil-soaked wiring and corroded connector at the solenoid, the single most common cause on higher-mileage cars where the loom sits near a leaking cam cover
- 2. Faulty solenoid with an open or shorted internal coil, or it's gummed up and sticking mechanically
- 3. Low or sludged engine oil, the VVT system runs on oil pressure so a neglected service history shows up here fast
- 4. Poor earth or a blown fuse on the solenoid control circuit, often overlooked
- 5. Open circuit, short to earth, or short to power in the harness, frequently intermittent and only shows up when the loom flexes or heats up
- 6. ECM fault or a software glitch, rare and only worth considering once everything else checks out
How mechanics typically diagnose
A typical diagnostic sequence used by mechanics, provided here for educational reference only. Diagnostic work should be performed by a qualified mechanic with the appropriate tools and training.
- 1. Plug in a scanner and read the full code list plus the freeze-frame data from when P0078 logged. Note any timing codes (P0011, P0014) sitting alongside it, because they point at the same VVT system
- 2. Pull the connector off the solenoid and look hard at it. Oil contamination, green corrosion on the pins, a spread terminal, these false this code constantly and cost nothing to find
- 3. Check the oil level and condition before you touch anything electrical. Black, thin or low oil can throttle the VVT system and trigger this without a single failed component
- 4. Measure the solenoid coil resistance with a multimeter. Most read somewhere in the 5 to 15 ohm range, but check your make's figure. Open circuit or way out of spec means the solenoid is done
- 5. Back-probe the connector with the ignition on and confirm you have a clean supply voltage and a solid earth. No power or a dodgy earth sends you back up the loom, not to the solenoid
- 6. Clear the code and drive it. If it comes straight back, suspect the solenoid or a hard fault. If it takes a while or only returns over bumps, you're chasing an intermittent wiring break
Common questions about P0078
How long should this take a garage to sort out? +
If it turns out to be a corroded connector or a wiring repair, a decent tech will diagnose and fix it inside an hour or two. Swapping the solenoid itself is usually a one to two hour job depending on how buried it is, and some engines have it tucked behind the timing cover area which adds time. The diagnosis is the bit that varies. Chasing an intermittent wiring fault can eat a couple of hours on its own, so expect to pay for diagnostic time before any parts go on.
Is a cheap aftermarket solenoid worth fitting or should I go OEM? +
For VVT solenoids I'd lean towards OEM or a known quality brand. The cheap eBay ones are hit and miss, and a solenoid that sticks or has the wrong coil resistance will just relog the code or, worse, mess up your valve timing. A genuine or reputable aftermarket part is typically in the low to mid three figures and worth the difference here. This isn't a part to gamble on with a no-name special.
Can I keep driving with P0078 showing? +
Short term, usually yes, but listen to the car. If it's only the light on and it drives normally, you can get to a garage without drama. If it's gone into limp mode, stalling, or feeling really gutless, don't flog it, because running with the valve timing stuck wrong can hammer fuel economy and put extra strain on the engine. And if low oil is the cause, every mile you do unaware is doing damage.
Will it stop me passing the MOT? +
The code on its own isn't an MOT line, but the engine management light is. If the MIL is lit when the tester looks at it, that's a fail on the warning lamp check on any car built to display it. Sort the underlying fault, clear the code, and drive a few cycles so the light stays off before you book the test.
Information only, not professional advice
The information on this page is provided for general guidance and educational purposes only. It is not a substitute for diagnosis or repair advice from a qualified mechanic. Always verify any fault before paying for repairs. carfaultcodes.co.uk accepts no liability for decisions made based on this information. Full terms →