P24F5
NOx Sensor Delayed Response - High to Low Bank 1 Sensor 2
The post-catalyst NOx sensor on bank 1 (sensor 2) reacts too slowly when NOx in the exhaust drops from a high to a low level. Same failure mechanism as the upstream sensor — aged element, soot or condensate — but post-catalyst the consequence is a degraded SCR conversion-efficiency monitor.
- 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 B1s2
P24F5 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.