P21B5: NOx Adsorber - Over Temperature (Bank 2)

What does P21B5 mean?

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.

The code tells you which monitor failed, not which part is broken. Test before replacing parts.

Symptoms

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

Common causes

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

Likelihood is a general estimate across vehicles; your make and model may differ.