OBD-II code · Powertrain (generic, extended)
P21B5 NOx Adsorber Over Temperature (Bank 2)
A temperature sensor at the NOx adsorber on bank 2 reports a value above the safe limit. Sustained over-temperature destroys the storage washcoat.
Is it safe to drive with P21B5?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyRunaway desulphation regeneration
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
- Hard
- DIY friendly
- Best left to a mechanic
- Typical labour
- 1.5–8 h
- Emissions test
- Likely to fail
- Parts involved
- Nox Adsorber Catalyst
Related codes
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.