OBD-II code · Powertrain (generic, extended)
P24F4 NOx Sensor Delayed Response - High to Low Bank 1 Sensor 1
The pre-catalyst NOx sensor on bank 1 (sensor 1) reacts too slowly when NOx in the exhaust drops from a high to a low level — for example after a fuel cut or transition into low-load. Aged ceramic cells, soot deposits or condensation inside the sensor protective tube slow the diffusion of NOx to the measuring element.
Is it safe to drive with P24F4?
Usually safe to drive in the short term. Get it diagnosed when convenient, sooner if the car drives differently.
Common causes
- Most likelyAged sensor element with reduced response speed
- PossibleSoot clogging the protective tube
Symptoms
- Check engine light or NOx warning on
- AdBlue/DEF level or quality warning
- Possible no-start countdown after ignore period
- Failed emissions inspection (NOx)
Repair at a glance
- Difficulty
- Medium
- DIY friendly
- Best left to a mechanic
- Typical labour
- 0.6–2 h
- Emissions test
- Likely to fail
- Parts involved
- Nox Sensor B1s1
Related codes
- P24F5NOx Sensor Delayed Response - High to Low Bank 1 Sensor 2
- P24F1Control Module Wake-up Circuit High
- P24F2EGR Temperature/Charge Air Cooler Temperature Correlation
- P24F3Reductant Tank Temperature/Fuel Temperature Correlation
- P24F6Exhaust Aftertreatment Fuel Air Purge Valve Stuck Open
- P24F7Exhaust Aftertreatment Fuel Air Purge Valve Stuck Closed
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.