Powertrain, generic (extended)

P23B8 NOx Sensor Heater Sense Circuit Low (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

Smart NOx sensors report heater temperature through a separate sense connection, which GM calls the HTemp pin. P23B8 means this line on sensor 3 of bank 1 is pulled to ground. GM's 2025 table 25OBDG04B looks for the ground short on the post-second-catalyst sensor once the powertrain relay is above 11 V, the relay feeding the sensor bus is switched on, the supply exceeds 9.90 V and the dew point is reached; 80 of 160 samples at 25 ms must fail, Type B with two trips. For Continental Gen 3.5 sensors GM adds that the sensor itself waits 1.5 s before passing on an electrical fault. Bulletin 21-NA-149 names P23B8 where the engine harness of 3.0L LM2 trucks has rubbed through. P23B9 is the short to power; P2210 and P22A8 cover sensors 1 and 2.

Likely causes

Sense wire chafed through and touching body, frame or exhaust ground
Water or salt bridging the sense pin to ground in the connector
Internal short in the sensor or its control unit

Symptoms

  • Check engine light after two trips
  • Sensor 3 does not report valid NOx values
  • Often stored together with other wiring-related codes

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

P23B9P23B4P23BAP2210P22A8

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