P209E
Reductant Injection Air Pressure Sensor A/B Correlation
The two reductant injection air-pressure sensors A and B disagree by more than the allowed tolerance — one is reading off, or there is a real pressure imbalance between the two air-assist branches.
- 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.
- One sensor drifts relative to the otherCommon
- Real imbalance between air branchesPossible
Parts involved
- Reductant Air Pressure Sensor
- Reductant Air Pump
P209E 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.