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.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: hard
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.
- LNT thermally agedCommon
Parts involved
- NOX Adsorber Catalyst
P21B3 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.