Powertrain, generic (extended)

P23B4 NOx Sensor Heater Sense Circuit (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

Besides its drive line, the heater of a NOx sensor has a sense connection through which its resistance, and with it the element temperature, is measured. P23B4 reports a fault in that sense path for the third sensor on bank 1. GM's 2025 diesel table 25OBDG04B checks the HTemp pin of the post-second-catalyst sensor for an open circuit: 80 of 160 samples, Type A, MIL after one trip. A note in the table says that the Continental Gen 3.5 sensors debounce electrical faults internally for 1.5 s before reporting them, which lets the affected wire be pinpointed. Bulletin 21-NA-149 adds P23B4 to the harness-chafe list of LM2-powered 3.0L trucks from 2021–2022. Heater circuit monitoring is a CARB requirement. P23B5 is the plausibility check, P2208 and P22A6 the other sensors.

Likely causes

Heater sense lead broken between probe and module
Backed-out or corroded terminal at the probe connector
Internal break in the sensor 3 probe

Symptoms

  • Check engine light after one trip
  • Sensor 3 temperature status invalid

Repair estimate

DifficultyMediumDIY possible
Parts and labour€100–1,000
Time0.5–2.5 h

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

Parts involved

nox sensor b1s3 heater

Related codes

P23B5P23B8P23B9P23BAP2208P22A6

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