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. P04AA means valve B's position signal jumps or drops out sporadically; Bentley stores sporadic bank 2 sensor faults as P04A8 instead. No manufacturer document available to us uses this code; the description mirrors the valve A counterpart P048F and CARB's monitoring requirement for input and output components.
Medium severity. Can I keep driving?Drive as little as possible and gently. If the transmission slips or stays in one gear, go slowly to a repair shop or call for a tow truck.
Symptoms
- Check engine light on
- Flap without function, exhaust sound changed
- VW/Audi TDI: EGR fault P0401 possible
Most likely causes
- Loose contact or corrosion at the actuator connector
- Wire broken by vibration or heat
- Position sensor worn
What to check, in order
Start with the simplest. You can tick off each step.
Good to know
- Repair difficulty: medium
- It affects emissions.
What each character of P04AA means
- P: Powertrain: engine and transmission
- 0: Generic code: same on every make
- 4: Emissions
- AA: Specific fault in that system
Frequently asked questions
Can I keep driving with code P04AA?
Drive as little as possible and gently. If the transmission slips or stays in one gear, go slowly to a repair shop or call for a tow truck.
What causes code P04AA?
The most common causes are: loose contact or corrosion at the actuator connector; wire broken by vibration or heat; position sensor worn.
Related codes
- P04A6Exhaust Pressure Control Valve B Position Sensor/Switch Circuit
- P04A7Exhaust Pressure Control Valve B Position Sensor/Switch Circuit Range/Performance
- P04A8Exhaust Pressure Control Valve B Position Sensor/Switch Circuit Low
- P04A9Exhaust Pressure Control Valve B Position Sensor/Switch Circuit High
- P048FExhaust Pressure Control Valve A Position Sensor/Switch Circuit Intermittent/Erratic
Sources
Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.