Powertrain, generic (extended)

P23B9 NOx Sensor Heater Sense Circuit High (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

P23B9 is the counterpart of P23B8: the temperature feedback wire of NOx sensor 3 on bank 1 carries a voltage that only a short to battery or to another live wire can explain. The sensor then cannot report a trustworthy heater temperature, so it withholds its NOx output. GM applies this check to the post-second-catalyst sensor in 25OBDG04B (2025) under the same frame as the ground short: powertrain relay above 11 V, sensor supply above 9.90 V, dew point reached, and half of 160 samples (80) must fail before this Type B code is stored. A harness overheated near the exhaust, where insulation melts between neighbouring wires, is a classic trigger; the same 21-NA-149 bulletin for the 3.0L LM2 includes P23B9. P2211 and P22A9 cover sensors 1 and 2.

Likely causes

Sense wire shorted to the heater supply or another battery-fed wire
Harness insulation melted by contact with hot exhaust parts
Internal fault in the sensor or its control unit

Symptoms

  • Check engine light after two trips
  • No NOx reading from sensor 3
  • Possibly a blown fuse in the sensor supply

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–900
Time0.5–3 h

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

Parts involved

nox sensor b1s3 heater

Related codes

P23B8P23B4P23BAP2211P22A9

References: gsi.ext.gm.com, static.nhtsa.gov, law.cornell.edu.