Powertrain, generic (extended)

P21B5 NOx Adsorber - Over Temperature (Bank 2)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

P21B5 is the bank 2 version of the adsorber over-temperature fault and therefore only exists where each cylinder bank feeds its own NOx storage catalyst, which in practice means V engines with two exhaust paths. If only bank 2 overheats, the cause most likely sits on that side: an injector delivering too much, misfire limited to that bank, or a faulty temperature sensor there. If P21B4 is stored as well, look instead for something both banks share, such as the regeneration strategy, fuel quality or a skewed air mass reading. Eaton patent US7251929 puts the ageing limit near 750 °C during desulphation. No manufacturer publishes its P21B5 limit; the duty to monitor adsorber performance comes from CARB (13 CCR 1968.2 (f)(8)). Driving on risks permanent damage.

Likely causes

Injector on bank 2 delivering too much fuel
Misfire confined to bank 2
Regeneration overheating both banks (with P21B4)
Bank 2 exhaust temperature sensor reading too high

Symptoms

  • Check engine light on
  • Reduced power to protect the catalyst
  • Uneven exhaust temperatures between the banks

Repair estimate

DifficultyHardWorkshop job
Parts and labour€150–4,000
Time1–6 h

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

Parts involved

nox adsorber catalystexhaust temperature sensor

Related codes

P21B4P2001P2429

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