P23AFNOx Sensor Circuit High (Bank 1 Sensor 3)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- How the car detects it
- 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.
- What you may notice
- Check engine light after two trips
- Sensor 3 shows implausibly high NOx even at idle
- SCR efficiency code may follow
- Likely causes
- LikelyWire broken inside the insulation near a connector in the exhaust sensor harness
- PossibleSensor 3 failed with a reading stuck high
- RareShort between sensor lines in the harness
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Often possible to do yourself.Typical cost: €50 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.