Powertrain, generic · Auxiliary emission controls

P04A8 Exhaust Pressure Control Valve B Position Sensor/Switch Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

An exhaust pressure control valve is an electrically driven flap in the exhaust with a position sensor or end-position switch, usually inside the actuator. Valve B is the second such flap; Bentley's V8 assigns A to bank 1 and B to bank 2. Documented uses are the flap behind the particulate filter of VW's and Audi's 2.0 TDI, which raises back pressure for the low-pressure EGR, and the sound flaps of Audi and Bentley. P04A8 means valve B's position signal is below its window. Bentley's Continental and Flying Spur V8 of 2023 report it for bank 2 together with P04A0 as a sensor short to ground, often stored as sporadic; wiring and contacts are checked, otherwise the actuator is replaced and the basic setting carried out.

Likely causes

Signal wire shorted to ground (chafed at the underbody)
Position sensor or actuator faulty internally
5 V sensor supply open or shorted
Moisture or corrosion in the connector

Symptoms

  • Check engine light on
  • Flap without function, exhaust sound changed
  • VW/Audi TDI: EGR fault P0401 possible

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–900
Time0.5–2.5 h

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

Parts involved

exhaust pressure control valve b position sensorexhaust pressure control valve bsensor wiringengine control module

Related codes

P04A6P04A9P04A0P04AAP048D

References: static.nhtsa.gov, law.cornell.edu.