What it means
Same as P04A6 with intermittent or erratic feedback - typically a connector or harness fault. Wiring faults of this kind typically come from chafing, water ingress at connector seals, or pin push-back — a wiggle test on the harness while the code is active often confirms the location.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Exhaust Pressure Control Valve B changes exhaust backpressure to support engine warm-up or exhaust braking, while its position sensor reports the valve's movement to the control module. P04AA sets when the module sees erratic or interrupted position feedback from valve B instead of a stable signal that follows the command.
Common causes
Most likely first.
- Intermittent contact at connector
- Connector with intermittent contact under vibration
- Harness routed against vibrating component
Symptoms
- Check engine light on, possibly intermittent
- Intermittent changes in engine power or response
- Engine may take longer to warm up or have reduced performance
- Exhaust braking may work inconsistently, if equipped
How to diagnose it
- Check freeze-frame data and live valve B position feedback for dropouts or sudden jumps while the valve is commanded to move.
- Inspect valve B's connector and harness for loose or backed-out pins, moisture, corrosion, or insulation rubbed through against a vibrating part.
- With the code active, gently move the harness near valve B and watch live position data to see whether the signal cuts in or out.
- Check the valve B signal, reference, and ground circuits for intermittent opens or poor connections while moving the harness.
- If wiring and connections remain sound, test valve B's position sensor and valve response against the manufacturer's specification.
Common fixes
- Secure the valve B connector and repair loose or pushed-back terminals.
- Clean and reseal a moisture-affected connector, replacing damaged terminals as needed.
- Repair or reroute harness sections chafing against vibrating components.
- Replace the valve B position sensor or valve assembly if its feedback remains erratic with sound wiring.
Can I keep driving?
If the engine runs normally and the warning light is steady, a short, gentle trip may be possible, but avoid heavy loads and towing. Stop if power changes sharply or exhaust braking behaves unpredictably; continued driving can impair engine warm-up or braking control where fitted.
A code shows where the control module detected a fault, not always which part failed. Check the causes in order and confirm with a test before replacing parts. Have several codes? Check them together
This is a generic code: it means the same on every make. The car’s maker may add more detail in its own service information.