P0478 code meaning
Exhaust Pressure Control Valve A High
Powertrain system Generic code P04xx codes Check engine light
What P0478 means
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.
P0478 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modePossible
- Repair difficultyMedium
- Typical labor0.5 to 2.5 hours
- Clears after repairAfter a drive cycle
Symptoms of P0478
- Check engine light on
- Flap without function: sound unchanged, rattling
- Diesel: longer warm-up, exhaust brake weak or EGR faults
Common causes of P0478
- Most likelyFlap actuator electronics faulty
- PossibleControl wire shorted to battery voltage
- PossibleConnector damaged by moisture or heat
- Less likelyDriver in the engine module
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Exhaust pressure control valve aValve actuatorActuator wiringEngine control module
What each part of P0478 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 0 | Generic code: it means the same on every make (SAE standard). |
| 4 | Area: auxiliary emission controls (catalytic converter, EGR, EVAP). |
| 78 | The exact fault in that area. |
What to do next
- Note P0478 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P0478
What does P0478 mean?
P0478 means "Exhaust Pressure Control Valve A High". 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. It is a generic code, so it means the same on every make.
Is it safe to drive with P0478?
Drive gently and only to a workshop. With P0478 the car can go into limp mode (reduced power) to protect itself.
Will P0478 fail an emissions test?
Yes, it can. P0478 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P0478 to fix?
Difficulty: medium. A typical repair takes 0.5 to 2.5 hours of work. This is a job for a workshop.
Will P0478 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.