P21B4
NOx Adsorber Over Temperature (Bank 1)
A temperature sensor at the NOx adsorber on bank 1 reports a value above the safe limit. Sustained over-temperature destroys the storage washcoat.
- 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.
- Runaway desulphation regenerationCommon
Parts involved
- NOX Adsorber Catalyst
- Exhaust Temperature Sensor
P21B4 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.