P20AAReductant Injection Air Pressure Control Valve Stuck Open
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P20AA describes a mechanical condition: the air pressure control valve lets air through even when it is commanded closed or to a lower setting. The air pressure sensor then shows pressure in phases where it should have fallen, for example after shutdown or during dosing pauses. Constant airflow can push DEF toward the nozzle unmetered, cool the mixing area and waste compressed air, which on trucks also strains the brake air supply. No manufacturer documents P20AA publicly; the closest documented case is Deutz's own code P1126 for a defective SCR air valve. Distinguish it from P20AB, where the valve blocks the air, and from P20A7, where the pressure is merely off target. Dirt, water or oil from the compressed-air system holding the valve off its seat is the usual reason.
- What you may notice
- SCR warning lamp
- Air pressure remains present with the valve commanded closed
- Audible air flow at the dosing module, compressor runs more often
- Likely causes
- LikelyDebris, water or oil from the air system on the valve seat
- PossibleWorn valve seal or spring
- RareValve held open electrically by a wiring or driver fault
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €80 – €500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.