Powertrain, generic (extended)

P23C2 NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 3)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P23C2 targets the dead time of NOx sensor 3 on bank 1 when NOx drops: the reading only starts to fall too long after the exhaust has changed, even if it then moves at a normal pace. CARB separates these two failure types in its oxygen sensor definitions, calling the time before a sensor first reacts delayed response and a sluggish transition slow response. No manufacturer documents P23C2 publicly. GM's 2025 table builds such a delay timer into the front-sensor test P22F9 for the rising edge: it runs from the moment the NOx model reaches 30 ppm until the sensor does, and more than 2 s counts as a fault. A partly blocked protective tube or aged cells are the usual reason. Sensors 1 and 2 use P24F4 and P24F5, while P23C0 stands for a slow falling transition.

Likely causes

Protective tube partly blocked, so new exhaust gas reaches the cell late
Aged sensing cells reacting late to a change
Condensate in the sensor housing after many short trips

Symptoms

  • Check engine light on
  • Sensor 3 value holds briefly before dropping in overrun
  • Engine runs normally

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

P23C0P23BFP24F4P24F5

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