P21B2Excessive Time To Enter Closed Loop NOx Adsorber Control
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Needs a drive cycle to confirm a fix
- How the car detects it
- P21B2 is set when the feedback control of a NOx adsorber (lean NOx trap) has not become active within the time the manufacturer allows once its enable conditions are met. CARB requires this check for vehicles with feedback control of the adsorber or its active injection (13 CCR 1968.2 (f)(8.2.3)(A)(i)). Cummins describes such a loop in patent US7594392: oxygen sensors before and after the adsorber, open-loop rich/lean cycling first, then a closed loop that holds the bed near 650 °C during regeneration. Ram 2500/3500 trucks with the 6.7L Cummins (2007–2011) carry a NOx absorber; Chrysler bulletin 25-001-13 links its P2000 to fouled oxygen sensors. No maker publishes criteria for P21B2; slow sensors or a cold exhaust are the usual suspects.
- What you may notice
- Check engine light on
- Regeneration of the NOx trap delayed or skipped
- Higher NOx emissions, possible test failure
- Likely causes
- LikelyFouled or slow oxygen sensor before or after the adsorber
- PossibleExhaust not reaching control temperature (short trips, low load)
- RareExhaust leak ahead of the sensors
- RareExhaust temperature sensor reporting too low
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Medium · Usually a job for a workshop.Typical cost: €100 – €2,500
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.