Powertrain, generic (extended)

P23C1 NOx Sensor Performance - Sensing Element (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P23C1 asks whether the measuring cells of NOx sensor 3 on bank 1 have lost sensitivity, for example because deposits clog the internal cavities. GM's 2025 table 25OBDG04B runs the test on the post-second-catalyst sensor during afterrun: the sensor shifts its internal pump currents so that oxygen in the second cavity rises to a target, and the outcome is set against a value the supplier recorded on the new part and wrote into the sensor EEPROM. A ratio above 125 % or below 75 % fails; results are smoothed with EWMA logic, the check runs once per trip at engine speeds under 1,500 rpm and exhaust temperatures below 400 °C, with Type A rating and a one-trip MIL. GM bulletin 21-NA-149 also lists P23C1 on 3.0L LM2 trucks. P22FB and P22FE are sensors 1 and 2.

Likely causes

Internal measuring cavities clogged by ash or other deposits
Sensing cells worn out after high mileage
Wiring fault upsetting the sensor supply during the test (GM 3.0L LM2)

Symptoms

  • Check engine light after one trip
  • SCR monitoring results become unreliable
  • No noticeable change in engine behaviour

Repair estimate

DifficultyMediumDIY possible
Parts and labour€150–900
Time0.5–2 h

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

Parts involved

nox sensor b1s3

Related codes

P23BCP23BDP23BEP22FBP22FE

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