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.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: medium
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)
Common causes
Ordered by how often each turns out to be the cause.
- Aged sensor element with reduced response speedCommon
- Soot clogging the protective tubePossible
Parts involved
- NOX Sensor B1s1
P24F4 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.