P22FA: NOx Sensor Performance - Slow Response High to Low Bank 1 Sensor 1
What to do
Ask whether the engine is using oil, and what kind of journeys the car does. Both coat this sensor and slow it down — and a replacement fitted to the same engine is coated the same way, so the cause matters as much as the part.
Ask for both NOx sensors to be compared before either is replaced. These are expensive, and the comparison is what identifies which is at fault.
Do not let an SCR catalyst be replaced on the strength of an efficiency code alone if this is stored too. A slow sensor makes the system dose late, which looks exactly like a failed catalyst — and the catalyst is far more expensive.
Occasional longer runs genuinely help, because the sensor needs sustained heat to keep its element clean.
What this code means
The upstream NOx sensor takes too long to follow a fall in nitrogen oxide.
Slow response is a different measurement from a biased reading, and the difference decides what is wrong. A biased sensor reports the wrong number. A slow one eventually reports the right number, late.
The direction is specified here, and it is diagnostic. High to low is the sensor following NOx falling — which happens when the engine goes off load, or when exhaust gas recirculation opens and suppresses NOx formation. A sensor that lags in this direction specifically is usually one whose element has been coated.
Deposits act as a barrier. Gas has to diffuse through them to reach the sensing element, so changes arrive blurred and late while a steady reading still settles correctly. That is exactly the signature of a fouled sensor rather than a failed one.
What coats it is worth pursuing, because a replacement meets the same conditions:
- Oil ash, from an engine burning oil.
- Soot, from short journeys where the sensor never runs hot enough to keep itself clean.
- Sulphur, from persistent use of poor fuel.
Ageing is the honest answer on a high-mileage sensor, and these do wear out. But a sensor slow well before its time is reporting on the engine in front of it, not on itself.
Dosing accuracy is what suffers. The system meters reductant from this signal, so a lagging sensor doses late — which shows up as poor SCR efficiency without the catalyst being at fault.
Likely causes
Start here
- Contaminated oxygen sensor element · moderate
- Faulty NOx sensor · moderate
- Engine burning oil into the exhaust · specialist
Less common
- Repeated cold starts on very short journeys · easy
- Contaminated or poor quality fuel · moderate
Symptoms
Components involved
Information, not instruction
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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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What makes one worth reading:
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