Fault codeP0478

MODERATE

Exhaust Pressure Control Valve High

What this code means

Exhaust Pressure Control Valve High.

P0478 is a generic code: it means the same thing on every make. P0478 belongs to family P04xx — Emission controls. Its page explains how to read a code and groups all its codes by part.

Type of fault

The control unit reads a voltage on this circuit above its expected range. Usual causes: a broken wire or a poor ground, a short to a supply voltage, or the component itself. As with a low signal, check the wiring and connector before the part.

Exhaust back-pressure sensor

On diesel engines and some others, a sensor measures the pressure in the exhaust, often before the particulate filter or the turbo. The control unit uses it to control the turbo, the EGR and, on some models, to estimate how clogged the filter is.

What you notice

Loss of power or limp mode; sometimes regenerations of the particulate filter that occur too often or not at all.

How to check it

The sensor is connected to the exhaust by a small pipe: check that it is not split, disconnected or blocked with soot. Then the connector and the wiring near the hot exhaust. Engine off, it should read close to the atmospheric pressure.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

Intake manifold pressure

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

How to read this code

PPowertrain
0standardised code
4Emission controls
78P0478