P0478Exhaust Pressure Control Valve A High
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- The exhaust pressure control valve is a flap in the exhaust: on older Navistar/Ford diesels a warm-up flap that raises back pressure when cold, on Ram and others an exhaust brake, on VW's 2.0 TDI a flap that drives the low-pressure EGR, and on Audi and Bentley a sound flap. P0478 means the control circuit of valve A is high – a short to positive, an open ground, or a driver reporting a high state. VW's 2.0 TDI of 2015 sets it together with P0475, P0477, P048B and P048E, and the actuator electronics are replaced. CARB requires output components to be monitored for circuit faults. The flap then no longer works.
- What you may notice
- Check engine light on
- Flap without function: sound unchanged, rattling
- Diesel: longer warm-up, exhaust brake weak or EGR faults
- Likely causes
- LikelyFlap actuator electronics faulty
- PossibleControl wire shorted to battery voltage
- PossibleConnector damaged by moisture or heat
- RareDriver in the engine module
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €150 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.