Powertrain, generic · Auxiliary emission controls

P04A4 Exhaust Pressure Control Valve B Stuck Open

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. P04A4 means valve B is commanded to close but its feedback stays open. With an EGR flap too little back pressure means too little EGR, with a sound flap the exhaust stays loud. No manufacturer document available to us uses this code; the description mirrors the valve A counterpart P047F and CARB's monitoring requirement for input and output components.

Likely causes

Flap or flap shaft corroded or coked (stiff)

Flap actuator (motor or gear) faulty or too weak

Linkage between actuator and flap broken or misadjusted

Position sensor reports wrongly

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€200–1,500

Time0.5–2.5 h

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

Parts involved

exhaust pressure control valve bvalve actuatoractuator wiringengine control module

Related codes

P04A5 P04A0 P049F P047F P04A7

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