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P20AA

Reductant Injection Air Pressure Control Valve Stuck Open

Reductant air-pressure control valve mechanically stuck open — air pressure cannot be reduced, dosing atomisation off-spec.

  • Check engine light: Yes
  • Emissions related: Yes
  • Repair difficulty: medium

Symptoms

  • Check engine light or NOx warning on
  • AdBlue/DEF level or quality warning
  • Possible no-start countdown after ignore period
  • Failed emissions inspection (NOx)

Common causes

Ordered by how often each turns out to be the cause.

  • Valve stuck openCommon
  • Carbon or debris buildup preventing free movementLess likely
  • Component wear with age - mechanical playLess likely

Parts involved

  • Reductant Air Pressure Control Valve

P20AA is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.