Powertrain, generic (extended)

P20AA Reductant Injection Air Pressure Control Valve Stuck Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Debris, water or oil from the air system on the valve seat
Worn valve seal or spring
Valve held open electrically by a wiring or driver fault

Symptoms

  • SCR warning lamp
  • Air pressure remains present with the valve commanded closed
  • Audible air flow at the dosing module, compressor runs more often

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–500
Time0.5–2 h

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

Parts involved

reductant air pressure control valve

Related codes

P20ABP20A7P20A6P2037

References: law.cornell.edu, serdia.deutz.com, energy.gov.