P0478 Code: Exhaust Pressure Control Valve A High
🟠 Moderate severity · drivable
Drivable, but a valve stuck closed can build too much backpressure and cut power; fix it soon.
What P0478 means
The engine computer sees the exhaust pressure control valve reading too high, meaning too much exhaust backpressure is being built. A high reading is often caused by a valve stuck closed, carbon buildup, or a blockage in the exhaust. Too much backpressure can reduce engine power and cause overheating in some cases.
Symptoms
- Check engine light is on
- Loss of engine power
- Poor fuel economy
- Engine feels restricted at higher RPM
Common causes
- most common: Valve stuck in the closed position
- common: Carbon buildup jamming the valve
- common: Faulty valve actuator or position sensor
- less common: Blocked exhaust or catalytic converter
How to fix it
| Fix | DIY? | Typical cost |
|---|---|---|
| Clean the valve and remove carbon buildup | Yes | $50–150 |
| Replace the exhaust pressure control valve | Yes | $150–400 |
| Inspect the exhaust for blockages | Mechanic | $100–300 |
Frequently asked questions
Can I drive with a P0478 code?
Yes, but if the valve is stuck closed, power will drop and the engine may run hot. Get it fixed before driving hard.
Why is too much exhaust backpressure bad?
High backpressure makes it harder for the engine to push exhaust out, which lowers power and can overwork the engine.
How much does it cost to fix a P0478?
Cleaning the valve costs $50–$150. Replacing it typically runs $150–$400.
Related codes
P0475
P0475 Code: Exhaust Pressure Control Valve A Malfunction
P0477P0477 Code: Exhaust Pressure Control Valve A Low
P0476P0476 Code: Exhaust Pressure Control Valve A Range/Performance
Updated 2026-08. This page is informational and not a substitute for professional diagnosis.