P21B4NOx Adsorber - Over Temperature (Bank 1)
- 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
- P21B4 reports that the NOx adsorber on bank 1 has run hotter than the manufacturer's protection limit. The risk is greatest during desulphation, when the adsorber is heated on purpose: Eaton patent US7251929 gives about 750 °C as the maximum desulphation temperature, beyond which the adsorber ages thermally, while Cummins aims for roughly 650 °C during regeneration (US7594392). Excess heat comes from too much fuel in rich phases, leaking injectors, unburnt fuel after misfires, or a temperature signal that reads wrong. No manufacturer documents its P21B4 limit publicly; the code can only occur on engines with such a catalyst, for instance the 2007–2011 Ram 6.7L Cummins. Act promptly: repeated overheating ruins the storage coating and leads to P2000.
- What you may notice
- Check engine light on
- Reduced power while the engine protects the catalyst
- Strong smell or heat from the exhaust
- Likely causes
- LikelyToo much fuel during regeneration or desulphation
- PossibleLeaking or dribbling injector on bank 1
- PossibleMisfire sending unburnt fuel into the exhaust
- RareExhaust 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.