What Does P0078 Mean?
This code indicates a general electrical malfunction in the exhaust variable valve timing (VVT) oil control solenoid circuit on bank 1. The ECM has detected abnormal voltage, resistance, or current conditions in the circuit controlling the exhaust camshaft phaser. This prevents proper exhaust valve timing optimization, causing poor idle quality, increased emissions, and reduced engine efficiency.
Common Causes
35%
Failed exhaust VVT solenoid with degraded coil resistance outside the 6.5-8.5 ohm specification
25%
Corroded or heat-damaged wiring between ECM and exhaust VVT solenoid, often due to proximity to the rear of the cylinder head
20%
Sludge-contaminated solenoid screen restricting oil flow and causing erratic electrical behavior from hydraulic feedback
15%
Intermittent connector contact at the exhaust VVT solenoid due to vibration or corrosion
5%
ECM driver circuit failure for exhaust VVT control
Diagnostic Steps
1
Measure the exhaust VVT solenoid coil resistance at its connector. Specification is typically 6.5-8.5 ohms at 68°F (20°C). Values outside this range confirm a failed solenoid.
2
Inspect the solenoid connector and wiring for damage, corrosion, or heat exposure. The exhaust side solenoid wiring is particularly susceptible to heat damage from the exhaust manifold.
3
Remove the exhaust VVT solenoid and inspect the screen filter for sludge contamination. Clean or replace the solenoid as needed. Check the oil feed port in the cylinder head for blockage.
4
With the solenoid disconnected, verify battery voltage is present on the supply wire with key on. Check the ECM control wire for continuity to the ECM and ensure no shorts to ground or power.
5
After repair, clear codes and use a scan tool to command the exhaust VVT solenoid through its range. Monitor exhaust cam retard angle to verify proportional response to commanded duty cycle.
Estimated Repair Cost
$80 - $400
Parts + labor, varies by vehicle and location
This code indicates a general electrical malfunction in the exhaust variable valve timing (VVT) oil control solenoid circuit on bank 1. The ECM has detected abnormal voltage, resistance, or current conditions in the circuit controlling the exhaust camshaft phaser. This prevents proper exhaust valve ...
The most common cause of P0078 (Exhaust Valve Control Solenoid Circuit (Bank 1)) is: Failed exhaust VVT solenoid with degraded coil resistance outside the 6.5-8.5 ohm specification
Typical repair costs for P0078 range from $80 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Exhaust VVT malfunction primarily affects emissions and fuel economy. While not immediately dangerous, extended driving may cause catalytic converter overheating due to improper exhaust valve overlap timing.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0078 to identify the root cause.
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Quick Info
Category
Powertrain
System
Variable Valve Timing (VVT) System
Difficulty
Type
Generic (SAE)
Recommended Tools
OBD2 Scanner
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