Powertrain, generic (extended)

P209C Reductant Injection Air Pressure Sensor B Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modePossible

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.

Likely causes

Signal wire chafed through and touching ground
5 V sensor supply missing, for example pulled down by another sensor on the same rail
Pressure sensor B internally shorted

Symptoms

  • MIL or SCR warning on
  • Live data shows pressure at or below zero regardless of air supply
  • DEF dosing switched off

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–450
Time0.5–2 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

reductant air pressure sensor

Related codes

P209AP209DP209BP2039

References: law.cornell.edu, serdia.deutz.com, mediathek-bosch-automotive.com.