Powertrain, generic (extended)

P209A Reductant Injection Air Pressure Sensor B Circuit

Check engine lightOn

EmissionsRelevant

Drive cycleNot required

Limp modePossible

Air-assisted DEF dosing, as in Bosch's first Denoxtronic generation of 2004, mixes the fluid with compressed air before it reaches the exhaust, and a pressure sensor confirms that this air is there. A second sensor B can sit in another air branch or serve as a cross-check for sensor A. P209A is the general circuit fault of sensor B, set when its signal or supply cannot be evaluated at all; P209C and P209D split off the clearly low and high readings. No manufacturer documents P209A publicly, and Deutz, whose air-assisted EMR L1 system has only one such sensor, uses the P2037 family for it. CARB counts air-assisted injection among the delivery functions that must be monitored. Without a usable reading the controller usually suspends dosing, NOx rises and a derate may follow.

Likely causes

Corroded or loose connector at sensor B, often from DEF residue or road spray

Reference voltage or ground of the sensor interrupted or out of range

Internal electronic failure of pressure sensor B

Input stage in the dosing control unit defective

Symptoms

Malfunction indicator lamp or SCR warning on
DEF dosing suspended, higher NOx output
Engine power derate or inducement countdown on trucks

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

P209B P209C P209D P209E P2037

References: law.cornell.edu, energy.gov, mediathek-bosch-automotive.com.