P209CReductant Injection Air Pressure Sensor B Circuit Low
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- How the car detects it
- P209C means the signal of the second air pressure sensor in an air-assisted DEF system has dropped below the lowest voltage a working sensor can deliver. With common three-wire pressure sensors this points to a signal wire shorted to ground, a lost 5 V reference or a sensor failed internally; with some input circuits an open signal line also reads low. The controller then cannot tell whether atomizing air is present. The opposite fault, a reading stuck above the valid band, is P209D, while P209B covers values that are electrically possible but implausible. No manufacturer documents P209C publicly; for sensor A, Deutz files the matching fault (short to ground or open) as P2039. Expect the dosing to be stopped and a warning on the dash; on heavy vehicles an inducement may follow.
- What you may notice
- MIL or SCR warning on
- Live data shows pressure at or below zero regardless of air supply
- DEF dosing switched off
- Likely causes
- LikelySignal wire chafed through and touching ground
- Possible5 V sensor supply missing, for example pulled down by another sensor on the same rail
- PossiblePressure sensor B internally shorted
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €60 – €450
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.