Powertrain, generic · Auxiliary emission controls

P0478 Exhaust Pressure Control Valve A High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

Flap actuator electronics faulty
Control wire shorted to battery voltage
Connector damaged by moisture or heat
Driver in the engine module

Symptoms

  • Check engine light on
  • Flap without function: sound unchanged, rattling
  • Diesel: longer warm-up, exhaust brake weak or EGR faults

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€150–900
Time0.5–2.5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

exhaust pressure control valve avalve actuatoractuator wiringengine control module

Related codes

P0475P0477P0479P048EP04A2

References: static.nhtsa.gov, law.cornell.edu, patents.google.com.