Powertrain, generic (extended)

P21B2 Excessive Time To Enter Closed Loop NOx Adsorber Control

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

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.

Likely causes

Fouled or slow oxygen sensor before or after the adsorber
Exhaust not reaching control temperature (short trips, low load)
Exhaust leak ahead of the sensors
Exhaust temperature sensor reporting too low

Symptoms

  • Check engine light on
  • Regeneration of the NOx trap delayed or skipped
  • Higher NOx emissions, possible test failure

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€100–2,500
Time0.5–4 h

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

Parts involved

nox adsorber catalystoxygen sensor

Related codes

P21B3P2000P2A00P2A01

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