Powertrain, generic (extended)

P21B3 Closed Loop NOx Adsorber Control At Limit - Too High

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P21B3 means the NOx adsorber feedback loop has reached its upper adjustment limit and still misses its target, the case CARB describes in 13 CCR 1968.2 (f)(8.2.3)(A)(iii). In practice the controller asks for more enrichment or longer rich phases than the calibration permits, because the downstream oxygen signal never shows the expected change or the bed stays below its target. Cummins patent US8065871 evaluates this upstream/downstream lambda difference to find the lean-to-rich transitions of the adsorber. Plausible reasons are a sulphur-loaded or aged adsorber, a lagging rear oxygen sensor, air leaks before the sensors, or fuel delivery that falls short. No manufacturer publishes thresholds for P21B3; read it together with P2000 and oxygen sensor codes.

Likely causes

NOx adsorber sulphur-loaded or thermally aged
Downstream oxygen sensor responding too slowly
Air leak in the exhaust ahead of the sensors
Too little fuel delivered for the rich phases

Symptoms

  • Check engine light on
  • Higher fuel consumption from long rich phases
  • Often stored together with P2000

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–3,500
Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

nox adsorber catalyst

Related codes

P21B2P2000P2A01

References: law.cornell.edu, patents.google.com, patents.google.com, en.wikipedia.org.