P0478–P04A2 Exhaust Pressure Control Valve A High
The ECU has logged a fault on P0478. The definition below is the exact wording the standard uses for this code.
Can you drive it? The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.
Which code is which
Same fault, different cylinder or bank — diagnosed the same way. Several set together usually means one shared cause.
| Code | Definition | Severity |
|---|---|---|
P0478 | Exhaust Pressure Control Valve A High | Investigate soon |
P04A2 | Exhaust Pressure Control Valve B High | Investigate soon |
The definition
P0478: Exhaust Pressure Control Valve A High.
P0478 is a powertrain code (the engine or gearbox), defined by the SAE standard - it means the same thing on any make of car, and any generic OBD-II reader will decode it.
What to check first
Cheapest and fastest checks first:
- Read the freeze-frame data stored with the fault - engine speed, load and temperature at the moment it logged tell you whether this happens cold, hot or under load.
- Check the connector at the part for corrosion, and that it is fully seated.
- Clear the code and see whether it returns, and under what conditions.
What actually causes this on a VAG
The opposite electrical complaint to the low-circuit fault: the control unit sees the exhaust pressure control valve's circuit sitting high, which on this kind of output means an open circuit - a broken wire, a plug that is not connected, a corroded terminal, or a failed winding inside the valve. The valve simply stays where it rests. As with the rest of this family the generic name covers several different components, so have the fault read with VAG-specific software that names the part on your engine before any work starts.
Most likely causes, in order
- Connector unplugged or not fully latched. Always the first check, particularly if anything has recently been done in that area of the engine. These plugs can sit in place looking connected while the locking tab has never clicked home. Refit it firmly and see whether the fault returns.
- Broken wire or a corroded terminal. Heat cycling backs terminals out of connector bodies and corrosion eats the crimp behind the pin. Look into the connector at the pins themselves and check for a wire that has pulled back out of the housing.
- Missing supply feed from a shared fuse or relay. The feed to a valve like this is often shared with other components. If several unrelated things stopped working at the same moment, check the fuse and the relay that feed them before suspecting any individual part.
- Failed winding inside the solenoid or actuator. A fair conclusion once the connector, the wiring and the supply have all been proved good - not before.
Worth knowing: If more than one actuator went dead at exactly the same moment, a shared fuse or relay is the cheap answer and the individual parts are almost certainly innocent. Check that before anything is ordered.
Reading the code is the easy part
A code names the circuit that complained. What tells you why is watching the sensor behind it while the engine runs — and that is where a code reader stops and a diagnostic tool starts.
VAGPULSE reads every module in a VW, Audi, Skoda or SEAT — not just the engine — shows the live measuring blocks behind this fault, gives you the freeze-frame conditions it was logged under, and clears it once you have fixed it. It also does what a generic OBD reader cannot on a VAG: service resets, coding, adaptations, and Stage 1–3 flashing with your original file saved first.
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Prefer to work on your phone? The VAGPULSE Android app is a free download and its demo mode runs against a sample car with no hardware at all, so you can see exactly what it reads before spending anything. Reading your car with it needs the VAGPULSE cable, which comes with the licence.
Common questions
What does P0478 mean?
P0478 means Exhaust Pressure Control Valve A High. It is logged when the ECU sees this circuit outside its expected operating window.
Can I drive with P0478?
The engine light will stay on and the car may run in a reduced-power mode. Read the freeze-frame data to see the conditions the fault was logged under.