OBD-II code · Powertrain (generic, extended)
P21B3 Closed Loop NOx Adsorber Control At Limit - Too High
NOx adsorber closed-loop trim has reached the upper authority limit — the controller cannot correct further. Typically a degraded LNT or stuck-rich engine condition.
Is it safe to drive with P21B3?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyLNT thermally aged
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
- 2–6 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.