P0078
Exhaust Valve Control Solenoid Circuit (Bank 1)P0078 is a generic OBD-II powertrain diagnostic trouble code: Exhaust Valve Control Solenoid Circuit (Bank 1). It is logged by the engine control unit when the powertrain monitor detects that a specific fault threshold has been exceeded — typically resulting in the malfunction-indicator lamp (MIL / check-engine light) being illuminated.
What P0078 means
P0078 is stored when the Engine Control Module (ECM) detects an abnormal voltage or resistance in the Exhaust Valve Control Solenoid circuit on Bank 1. In engines equipped with Variable Valve Timing (VVT), the ECM uses a Pulse Width Modulated (PWM) signal to drive the exhaust valve control solenoid, which regulates oil pressure flow to the exhaust camshaft actuator, allowing the ECM to advance or retard exhaust valve timing for optimised power, fuel economy, and emissions across the rpm range. P0078 is a general circuit fault — the signal is out of expected range but not specifically pinned to a high or low state (those are P0079 and P007A respectively). The most common cause is a failed solenoid coil, but damaged wiring, corroded connectors, oil contamination at the harness, or a rare ECM driver failure can all trigger this code. When the fault is active, the ECM is unable to command correct exhaust valve phasing, so the VVT system effectively defaults to a fixed timing position, resulting in reduced performance, rough idle, and increased fuel consumption. While the vehicle typically remains driveable, extended operation with uncorrected VVT faults can increase wear on the variable timing mechanism and, on some vehicles, result in secondary fault codes.
Common causes
Most-frequently reported root causes when P0078 is logged.
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1
Failed exhaust valve control solenoid with open or shorted internal coil winding
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2
Damaged, chafed, or broken wiring in the solenoid control harness
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3
Corroded, pushed-back, or oil-contaminated connector pins at the solenoid
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4
Short to ground or short to voltage in the solenoid signal wire
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5
ECM internal driver transistor failure (rare)
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6
Incorrect ECM software calibration misinterpreting normal voltage fluctuations
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7
Oil sludge blocking solenoid pintle causing mechanical jam and electrical back-pressure
Symptoms drivers notice
How to diagnose P0078
A typical diagnostic flow when this code is present.
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1
Connect an OBD-II scanner, record P0078 freeze-frame data, and check for companion VVT or camshaft position codes
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2
Visually inspect the exhaust valve control solenoid connector and wiring harness for corrosion, oil intrusion, broken pins, or chafing
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3
With the ignition off, measure solenoid winding resistance across the two connector terminals — typical healthy range is 6–15 ohms; an open (OL) or near-zero reading indicates a failed coil
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4
With the ignition on, verify the ECM supplies approximately 12 V on the power terminal and a good ground on the return terminal
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5
Using a professional bi-directional scan tool, command the Bank 1 exhaust solenoid active and listen for an audible click; a click with code still present suggests sludge-jamming rather than electrical failure
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6
Inspect the engine oil condition — heavily sludged oil can block solenoid passages; perform an oil service before condemning the solenoid
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7
If wiring, connector, and solenoid all test within specification, perform ECM output verification with an oscilloscope before considering ECM replacement
Related powertrain codes
Frequently asked questions
Can I drive with P0078 stored?
The vehicle is typically driveable but will exhibit reduced power and higher fuel consumption because the VVT system is locked out. Prolonged driving with uncorrected VVT faults can accelerate wear on the cam phaser mechanism, so the fault should be diagnosed and repaired promptly.
Will clearing the code make it go away permanently?
Clearing the code only removes it from memory temporarily. If the underlying electrical or mechanical fault is not repaired, the ECM will detect the abnormal solenoid circuit again within a short drive cycle and re-set P0078.
How does P0078 differ from P0079?
P0078 is a general circuit fault indicating the solenoid voltage or resistance is out of the acceptable range without specifying direction. P0079 is a more specific "circuit low" fault, meaning the ECM is measuring a voltage that is below the expected threshold, typically pointing to a short to ground or a failed solenoid pulling the signal line low.
Could low or dirty engine oil cause P0078?
Yes. The VVT solenoid relies on pressurised engine oil to actuate the cam phaser. Low oil level reduces actuating pressure, and sludge can mechanically jam the solenoid pintle, causing the ECM to see abnormal circuit behaviour. Always check oil level and condition before replacing the solenoid itself.
Disabling P0078 in software
RaceTune can permanently disable P0078 — and any other OBD-II diagnostic trouble code — on every ECU family we support. The monitor is disabled inside the ECU itself, so the fault stops being logged: the warning light stays off and the engine never enters limp mode for this code. The change is tied to your exact software version.
Software modifications affect emissions compliance and are not road-legal in many jurisdictions. RaceTune service files are intended for motorsport, off-road, and export use.
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