OBD-II code · Powertrain (generic, extended)
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.
Is it safe to drive with P21B4?
The car may go into limp mode (reduced power). Get it diagnosed before a long trip.
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, Exhaust Temperature Sensor
Related codes
- P21B5NOx Adsorber Over Temperature (Bank 2)
- P21B1Reductant Level Sensor C Circuit Intermittent/Erratic
- P21B2Excessive Time To Enter Closed Loop NOx Adsorber Control
- P21B3Closed Loop NOx Adsorber Control At Limit - Too High
- P21B6Cold Start Cylinder 1 Injection Timing
- P21B7Cold Start Cylinder 2 Injection Timing
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.