P23BFNOx Sensor Performance - Slow Response Low to High (Bank 1 Sensor 3)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P23BF concerns how quickly NOx sensor 3 on bank 1 follows a rise in NOx. The control module waits for an increasing transient and times the climb of the sensor signal against what a model predicts. No manufacturer publishes P23BF. GM's 2025 table demonstrates the method on the front sensor under P22F9: timing runs from 30 ppm up to 220 ppm, and the sensor fails when it trails by over 2 s or by more than a tenth of the model's time. For a probe behind two SCR catalysts this is harder, since little NOx is left to reach it. A Continental smart sensor is specified with a 33-66 % response time of 1300 ms when fresh and 1650 ms when aged. CARB counts response rate among NOx sensor performance faults. P22F9 and P22FC are sensors 1 and 2; P23C0 is the falling edge.
- What you may notice
- Check engine light on
- Sensor 3 signal trails the front sensor during acceleration
- No change in how the engine runs
- Likely causes
- LikelySoot, oil ash or urea residue blocking the gas inlet of the sensor
- PossibleMeasuring cells aged beyond their response specification
- RareElement running too cold because the heater is weak
- 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: €150 – €900
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.