Powertrain, generic (extended)

P2C79 NOx Adsorber - Sulfur Limit Exceeded

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P2C79 says the sulfur load held in the NOx adsorber (lean NOx trap) has reached the limit the engine computer allows. Sulfur from fuel and oil occupies the storage sites more firmly than NOx, so capacity shrinks until a desulfation is run: rich phases with the trap above roughly 550 °C, as a Volkswagen patent explains. A Ford patent models the stored sulfur and starts this cleaning once a calibrated amount is reached, naming typical values between 0.1 and 0.5 g per litre of trap. CARB requires the trap and its desulfation hardware to be monitored. No manufacturer documents P2C79 publicly; it suggests the load stayed high, for instance because the trap never got hot enough on short trips. P2000/P2001 judge NOx efficiency, P21B4 overheating. NOx output rises and fuel use may grow.

Likely causes

Desulfation never completes because the trap stays too cool (short trips, idling)
Fuel with high sulfur content (outside EU/US ultra-low-sulfur grades)
Increased oil consumption carrying sulfur-bearing additives into the exhaust
Faulty exhaust temperature or lambda signal that blocks or aborts the rich desulfation phases
Thermally aged trap whose sulfur can no longer be removed

Symptoms

  • Check engine light
  • Higher fuel consumption from repeated rich regeneration attempts
  • Occasional rotten-egg smell from the exhaust
  • Raised NOx emissions, possible failure at an emissions test

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€80–2,500
Time0.5–4 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

nox adsorber catalyst

Related codes

P2000P2001P21B2P21B3P21B4

References: law.cornell.edu, patents.google.com, patents.google.com.