P24F5NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 2)
- 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
- P24F5 is the same delay check for NOx sensor 2 on bank 1, which sits behind the SCR catalyst and judges its conversion. If its value lags when NOx at the outlet falls, the efficiency estimate is distorted and the SCR monitor could pass or fail wrongly, so the sensor must react in time. A late reaction means the first visible change comes too long after the expected drop. No manufacturer publishes P24F5. CARB separates delayed from slow response in its sensor definitions; GM's 2025 diesel table shows a 2-second delay limit in its front-sensor test P22F9 as a model for such timing. Hella points to ammonia slip and crystal residue as threats to the element. P22FD is the slow falling case; P24F4 sensor 1.
- What you may notice
- Check engine light on
- Possibly an SCR efficiency code alongside
- Likely causes
- LikelyDEF crystals or ammonia slip attacking the rear sensor element
- PossibleAged measuring cells of sensor 2 reacting late
- RareCondensate in the cold tailpipe area affecting the sensor
- 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.