What are the common causes of P0478?

A failed exhaust pressure control valve A reading high. A short to voltage on the valve A control wire. A chafed valve A harness touching power. A valve A actuator stuck closed. An ECM driver fault.

Read the valve A control voltage on a scan tool. Check the valve A control wire for a short to voltage. Inspect the valve A harness near power wires. Check the valve A actuator for sticking. Replace valve A if the fault stays. Clear the code and retest.

P0478

Exhaust Pressure Control Valve A High

System: Auxiliary emission controls (P code)
Lamp: Check Engine Light
Category: Powertrain - Auxiliary Emissions Controls

What it means

P0478 means Exhaust Pressure Control Valve A High.

Where it lives

For P0478, the description names the valve as the part involved.

Check these first

A failed exhaust pressure control valve A reading high
A short to voltage on the valve A control wire
A chafed valve A harness touching power
A valve A actuator stuck closed
An ECM driver fault

What to do next

Read the valve A control voltage on a scan tool
Check the valve A control wire for a short to voltage
Inspect the valve A harness near power wires
Check the valve A actuator for sticking
Replace valve A if the fault stays
Clear the code and retest

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Updated

Generic SAE J2012 definition. Last updated 2026-10-10.