Powertrain, generic (extended)

P21B4 NOx Adsorber - Over Temperature (Bank 1)

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modePossible

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.

Likely causes

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

Symptoms

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

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

P21B5P2000P2428P2080

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