What it means
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.
At a glance
- Usually lights the check-engine light
- Affects emissions; the car can fail an emissions test
How it works
Reductant air pressure sensors A and B monitor pressure in two branches of the air-assisted DEF dosing system. The control module sets P209E when their readings differ beyond the allowed correlation, which can result from a drifting sensor or a real pressure imbalance between the branches.
Common causes
Most likely first.
- One sensor drifts relative to the other
- Real imbalance between air branches
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
- Review freeze-frame data and graph live readings from sensors A and B under the same operating conditions to confirm when and how far they diverge.
- Inspect both sensor connectors and harnesses for corrosion, loose terminals, damage, or routing that could affect one sensor's signal.
- Compare each sensor's signal and electrical supply with the manufacturer's specification to identify a sensor or circuit reading that is out of line.
- Check both air-assist branches for leaks, restrictions, or other pressure differences that could make valid sensor readings disagree.
- If the wiring and branches check out, test sensors A and B under matching conditions to identify a sensor that drifts.
Common fixes
- Repair a confirmed leak or restriction causing unequal pressure between the air branches.
- Clean corroded connections or repair damaged wiring at the affected sensor.
- Replace the sensor that fails testing or consistently disagrees under matched conditions.
- Repair any confirmed air supply or branch fault affecting pressure balance.
Can I keep driving?
You can often drive while arranging diagnosis, but incorrect DEF dosing can keep emissions above limits and may start a no-start countdown. Check for countdown messages and avoid delaying repairs.
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.