Powertrain, generic (extended)

P23AF NOx Sensor Circuit High (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleNot required
Limp modeNo

With P23AF the third NOx sensor on bank 1 delivers a value above its measuring range. GM's 2025 diesel table 25OBDG04B sets the code when the raw reading of the post-second-catalyst sensor exceeds 2,990 ppm in 120 of 240 samples, provided the air system control is active or DEF dosing is ready; the rating is Type B, so the MIL follows on the second trip. For the 3.0L LM2 diesel in 2021–2022 light trucks, GM bulletin 21-NA-149 adds a practical hint: with P23AF the harness at the exhaust pressure differential sensor connector is checked by tugging on the single wires, because conductors may break there while still measuring fine, and extra tape is added afterwards. CARB requires detection of out-of-range NOx signals. P23AE is the low counterpart, P2203 the front sensor.

Likely causes

Wire broken inside the insulation near a connector in the exhaust sensor harness
Sensor 3 failed with a reading stuck high
Short between sensor lines in the harness

Symptoms

  • Check engine light after two trips
  • Sensor 3 shows implausibly high NOx even at idle
  • SCR efficiency code may follow

Repair estimate

DifficultyMediumDIY possible
Parts and labour€50–900
Time0.5–3 h

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

Parts involved

nox sensor b1s3

Related codes

P23AEP23ADP23ACP2203P22A1P20EE

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