Powertrain, generic (extended)

P23AD NOx Sensor Circuit Range/Performance (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P23AD flags the third NOx sensor on bank 1 as electrically intact but not trustworthy. GM's 2025 diesel table 25OBDG04B describes the check for its post-second-catalyst NOx sensor: the internal stability flags of the NOx and lambda current control are watched, and a flag counts as off when, for instance, the V2 cell voltage leaves a 40 mV band around its set point or the pump currents Ip1 and Ip2 change faster than allowed. If the off share exceeds 0.50 %, with fuel request and its rate steady, dew point passed and the heater on target, the fault matures after 2 of 2 samples; Type B, MIL after two trips. CARB demands monitoring of NOx sensor performance. P23AE and P23AF are hard out-of-range values, P2201 the front sensor.

Likely causes

Aged or poisoned sensing cell giving unstable pump currents
High contact resistance in the sensor 3 wiring
Exhaust leak near the sensor upsetting the measurement

Symptoms

  • Check engine light after two trips
  • Sensor 3 NOx value jumps in live data
  • SCR efficiency monitor may not complete

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

P23ACP23AEP23AFP2201P229F

References: gsi.ext.gm.com, law.cornell.edu.