P24F4NOx Sensor Delayed Response - High to Low (Bank 1 Sensor 1)
- 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
- P24F4 concerns the dead time of the upstream NOx sensor on bank 1 when engine-out NOx falls, typically on lift-off into overrun: the reading stays put too long before it starts dropping, even if the later descent is normal. CARB defines this split for exhaust sensors, delayed response being the wait before the first reaction and slow response a sluggish transition. No manufacturer documents P24F4 publicly. GM's 2025 diesel table shows the method for the rising edge: inside the front-sensor test P22F9 a timer runs from the NOx model reaching 30 ppm until the sensor does, and over 2 s is a fault. Hella names deposits and ageing as reasons for late NOx readings. P22FA is the slow falling case, P24F5 sensor 2.
- What you may notice
- Check engine light on
- Front NOx value lingers on lift-off
- Engine runs normally
- Likely causes
- LikelySoot or oil ash on the diffusion barrier of the front sensor
- PossibleAged measuring cells reacting late to falling NOx
- RareExhaust leak ahead of the sensor distorting the transient
- 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.