P23AF code meaning
NOx Sensor Circuit High (Bank 1 Sensor 3)
Powertrain system Generic code P23xx codes Check engine light
What P23AF means
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.
P23AF at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium, DIY possible
- Typical labor0.5 to 3 hours
- Clears after repairWhen cleared with a scanner
Symptoms of P23AF
- Check engine light after two trips
- Sensor 3 shows implausibly high NOx even at idle
- SCR efficiency code may follow
Common causes of P23AF
- Most likelyWire broken inside the insulation near a connector in the exhaust sensor harness
- PossibleSensor 3 failed with a reading stuck high
- Less likelyShort between sensor lines in the harness
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox sensor b1s3
What each part of P23AF means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| 3 | Area: ignition system or misfire. |
| AF | The exact fault in that area. |
What to do next
- Note P23AF and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P23AF
What does P23AF mean?
P23AF means "NOx Sensor Circuit High (Bank 1 Sensor 3)". With P23AF the third NOx sensor on bank 1 delivers a value above its measuring range. It is a generic code, so it means the same on every make.
Is it safe to drive with P23AF?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P23AF fail an emissions test?
Yes, it can. P23AF is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P23AF to fix?
Difficulty: medium. A typical repair takes 0.5 to 3 hours of work. A careful DIY repair is possible with basic tools and a scanner.
Will P23AF clear itself after the repair?
Clear the code with a scanner after the repair. If the fault is fixed, it does not come back.