P0078 Exhaust Valve Control Solenoid Circuit Bank 1

P0078 points to an exhaust valve control solenoid circuit fault on bank 1. The exhaust VVT solenoid circuit is not operating normally.

Code details

Generic OBD-II · P0

System: Fuel / Air
Code type: Generic (SAE / ISO controlled)
Standard: SAE J2012 / ISO 15031-6

Diagnostic boundary

A diagnostic trouble code reports that a monitored test failed. It does not prove one specific part is bad, and manufacturer logic can add vehicle-specific conditions.

Do not replace exhaust cam phasers before testing solenoid control, oil condition, and circuit integrity.

Symptoms of P0078

Check-engine light on, sometimes with rough or uneven running.
Hesitation, surging, or a noticeable change in fuel economy.
Occasionally hard starting or a smell of unburnt fuel.

Common causes of P0078

A failing or contaminated sensor (MAF, MAP, oxygen or air-fuel sensor).
Intake or vacuum leaks, or an exhaust leak near an upstream sensor.
Wiring, connector, or fuel-delivery problems that skew the reading.

Useful checks before replacing parts

Check bank 1 exhaust solenoid feed, ground or driver command, resistance, connector fit, and circuit continuity under load.
Inspect oil condition, solenoid screen blockage, connector oil intrusion, harness routing near heat, and related cam timing codes.
Compare commanded exhaust cam timing with actual cam angle, RPM, load, and oil temperature when the fault sets.

Related OBD-II codes

These nearby fuel / air entries can help compare wording and pin down the fault area.