P2000: NOx Adsorber Efficiency Below Threshold — Bank 1
What to do
Before agreeing to replace anything, ask two things: whether the NOx sensors were verified, and whether a desulphation or forced regeneration was attempted. Both are far cheaper than the trap and both fix a real share of these. If most of your journeys are short and local, mention it — this device needs a properly hot exhaust to clean itself, and a pattern of short trips is a genuine cause rather than an excuse.
What this code means
The NOx adsorber is no longer storing and converting oxides of nitrogen as effectively as it should. This is a lean-burn aftertreatment device — a NOx trap — fitted where an ordinary three-way catalyst cannot work, which on a diesel or a lean-burn petrol engine is most of the time.
It works in two alternating phases. Running lean, it stores NOx chemically on its surface. Periodically the engine management deliberately runs rich for a few seconds to purge what has been stored, converting it to nitrogen. Both phases have to work, and the code says the pair are no longer achieving the conversion the module expects.
What makes this worth understanding before accepting a quote is that a trap rarely fails on its own. It is poisoned or it is not being regenerated, and both have causes upstream:
- Sulphur poisoning from persistently high-sulphur fuel, which is reversible with a high-temperature desulphation cycle rather than replacement.
- Purge cycles not completing, because the engine never reaches the conditions they need. Short journeys that never warm the exhaust properly are the usual reason, and the trap is then reporting a usage pattern rather than a defect.
- The sensors that judge it — the NOx sensors either side — reading incorrectly, in which case the trap is fine and the measurement is not.
A new trap fitted to a car that does short cold journeys will fail again on the same schedule.
Likely causes
Start here
- Sulphur poisoning of the NOx adsorber · moderate
- Purge cycles not completing · moderate
- Faulty NOx sensor · moderate
- Predominantly short journeys · easy
Less common
- Exhaust leak before the adsorber · moderate
- Failed NOx adsorber · specialist
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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