P21B4: NOx Adsorber - Over Temperature (Bank 1)

What does P21B4 mean?

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.

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

Symptoms

• Check engine light on
• Reduced power while the engine protects the catalyst
• Strong smell or heat from the exhaust

Common causes

• High Too much fuel during regeneration or desulphation
• Medium Leaking or dribbling injector on bank 1
• Medium Misfire sending unburnt fuel into the exhaust
• Low Exhaust temperature sensor reading too high

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