P2C79 code meaning
NOx Adsorber - Sulfur Limit Exceeded
Powertrain system Generic code P2Cxx codes Check engine light
What P2C79 means
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.
P2C79 at a glance
- Check engine lightYes, usually on
- Emissions testCan fail
- Limp modeNot expected
- Repair difficultyMedium
- Typical labor0.5 to 4 hours
- Clears after repairAfter a drive cycle
Symptoms of P2C79
- 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
Common causes of P2C79
- Most likelyDesulfation never completes because the trap stays too cool (short trips, idling)
- PossibleFuel with high sulfur content (outside EU/US ultra-low-sulfur grades)
- PossibleIncreased oil consumption carrying sulfur-bearing additives into the exhaust
- Less likelyFaulty exhaust temperature or lambda signal that blocks or aborts the rich desulfation phases
- Less likelyThermally aged trap whose sulfur can no longer be removed
Check the most likely causes first. Replacing parts by guesswork is the most expensive way to clear a code.
Parts involved
Nox adsorber catalyst
What each part of P2C79 means
| P | Powertrain system: engine, fuel system, ignition, emissions and transmission. |
| 2 | Generic code (second SAE range): it means the same on every make. |
| C | Area: hybrid propulsion. |
| 79 | The exact fault in that area. |
What to do next
- Note P2C79 and any other codes stored with it: several codes together often point to one cause.
- Check for loose connectors and damaged wiring around the parts the code names.
- Check open recalls for your car: some faults are fixed for free by the dealer. Enter your VIN.
- Clear the code only after the repair. If it comes back, the fault is still there.
Buying a used car? Scan it for stored codes before you pay. A recently cleared memory can hide a problem.
Questions about P2C79
What does P2C79 mean?
P2C79 means "NOx Adsorber - Sulfur Limit Exceeded". P2C79 says the sulfur load held in the NOx adsorber (lean NOx trap) has reached the limit the engine computer allows. It is a generic code, so it means the same on every make.
Is it safe to drive with P2C79?
Usually yes, for a short time, if the check engine light is steady and the car drives normally. If the light flashes or the car runs badly, stop and have it checked.
Will P2C79 fail an emissions test?
Yes, it can. P2C79 is emissions-related, so most inspection programs fail a car while it is stored or while the check engine light is on.
How hard is P2C79 to fix?
Difficulty: medium. A typical repair takes 0.5 to 4 hours of work. This is a job for a workshop.
Will P2C79 clear itself after the repair?
After the repair the car has to complete a drive cycle (a mix of city and highway driving) before the test runs again. Until then the readiness monitor shows "not ready", which can fail an inspection.