What it means
A general electrical fault is reported on reductant injection air-pressure sensor B — the second air-assist pressure sensor on dual-bank or dual-injector DEF systems.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant injection systems use an air-pressure sensor to help the control module monitor air-assisted delivery of DEF or another reductant. Despite the supplied meaning referring to sensor B, the title identifies sensor A; P209A indicates a general circuit fault for reductant injection air-pressure sensor A, so the specific electrical failure needs diagnosis.
Common causes
Most likely first.
- Sensor failed
- Harness fault
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)
How to diagnose it
- Save freeze-frame data and check reductant-system warnings and live air-pressure sensor A data to see when the circuit fault is present.
- Locate sensor A using the vehicle's service information and inspect its connector and harness for loose connections, corrosion, fluid intrusion, or damage.
- Check the sensor supply, ground, and signal circuits against the manufacturer's wiring information and specification.
- With the circuit isolated as directed by the manufacturer, check for opens or shorts in the sensor A wiring.
- If the wiring and supply are correct, test sensor A and the reductant control module inputs according to the manufacturer's procedure.
Common fixes
- Reconnect or clean the sensor A connector if it is loose or corroded.
- Repair an open, shorted, or damaged wire in the sensor A circuit.
- Replace reductant injection air-pressure sensor A if it fails testing.
- Repair or replace the control module only if its circuit input is confirmed faulty.
Can I keep driving?
You may be able to drive while the vehicle remains operational, but heed DEF or NOx warnings and any displayed countdown because some vehicles can restrict or prevent restarting. Avoid ignoring the warning, since emissions monitoring and reductant delivery may be impaired.
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.