P21B5NOx Adsorber - Over Temperature (Bank 2)
- Generic OBD-II code · Powertrain
- Turns on the check-engine light
- Matters for the ITP
- Can trigger limp mode
- Needs a drive cycle to confirm a fix
- How the car detects it
- 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.
- What you may notice
- Check engine light on
- Reduced power to protect the catalyst
- Uneven exhaust temperatures between the banks
- Likely causes
- LikelyInjector on bank 2 delivering too much fuel
- PossibleMisfire confined to bank 2
- PossibleRegeneration overheating both banks (with P21B4)
- RareBank 2 exhaust temperature sensor reading too high
- Inspection (ITP)
- Matters for the ITP. While the check-engine light is on, the car fails the emissions check.
- Repair
- Hard · Usually a job for a workshop.Typical cost: €150 – €4,000
- Related codes
Explanation from obdex (CC0), exactly as the app shows it. Guidance only; confirm with a workshop.