OBD-II code · Powertrain (generic, extended)
P221B NOx Sensor 1/2 Correlation (Bank 2)
NOx sensors 1 and 2 on bank 2 disagree by more than the allowed tolerance over a drive cycle. One sensor drifts, or there is a real NOx gradient that exceeds the calibrated map.
Is it safe to drive with P221B?
Usually safe for short trips, but don’t ignore it: the underlying fault can get more expensive over time.
Common causes
- Most likelyOne sensor drifts
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 h
- Emissions test
- Likely to fail
- Parts involved
- Nox Sensor
Code data: OBDex open dataset (CC0). Labour times are typical ranges, not a quote.