P0478 Exhaust Pressure Control Valve High

P0478 points to a high condition in the exhaust pressure control valve circuit. The controller sees voltage or feedback above the expected range for the valve control path.

Code details

Generic OBD-II · P0

System: Emissions
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 the controller before checking the valve circuit for open grounds and short-to-voltage faults.

Symptoms of P0478

Check-engine light, often with no obvious change in how the car drives.
A failed emissions test or incomplete readiness monitors.
Sometimes a faint fuel-vapour smell (on EVAP codes).

Common causes of P0478

A worn catalytic converter or an oxygen/air-fuel sensor reading out of range.
EVAP leaks — a loose or failed gas cap, cracked hose, or purge/vent valve.
Exhaust leaks, fuel-trim problems, or a fault in a related monitor.

Useful checks before replacing parts

Review valve command, control voltage, current draw, driver status, freeze-frame load, and related high-input or control circuit codes.
Inspect for short-to-voltage faults, open grounds, connector corrosion, damaged actuator wiring, stuck valve load, and heat-related harness damage.
Command the valve and verify whether the control circuit and exhaust pressure response change as expected.

Related OBD-II codes

These nearby emissions entries can help compare wording and pin down the fault area.