P229F: NOx Sensor Circuit Range/Performance Bank 1 Sensor 2
What to do
Ask for both NOx sensors to be compared rather than this one judged alone. They are only meaningful as a pair, and the comparison is what shows which one is wrong — these are expensive parts and that check is quick.
Ask for an exhaust leak check near the sensor, since air getting in skews the reading with a healthy sensor.
Mention any exhaust work the car has had. A sensor not screwed fully home sits out of the gas flow and produces this.
Mention whether the engine uses oil. Oil ash contaminates these sensors, and a replacement fitted into the same exhaust is contaminated the same way — so the cause matters as much as the part.
What this code means
The downstream NOx sensor is reporting a value that is electrically valid but not credible.
The check that catches this is worth knowing because it costs nothing to repeat. After the vehicle has stood and the engine is restarted, both NOx sensors are reading the same gas before any dosing begins — so they should agree closely at that moment. A downstream sensor that disagrees with its upstream partner at a point where they must match has identified itself.
Range/performance also covers the physically impossible. A downstream sensor cannot legitimately report more NOx than the upstream one, since the catalyst can only remove it. A reading that does so is not a catalyst fault; it is a sensor fault.
The causes divide into the element and its situation.
- Contamination of the element by soot, oil ash or sulphur, which dulls it gradually.
- An exhaust leak near the sensor, admitting air and skewing the sample.
- A sensor not seated to depth, sitting out of the flow after exhaust work.
- Age. These run at around 800°C in a corrosive stream and do not last for ever.
Before replacing it, check what the upstream sensor is doing. The pair are only meaningful together, and this corpus records the expensive version of getting that wrong — a biased sensor convicting a healthy catalyst.
Ask about oil consumption, since a sensor contaminated early was contaminated by something that is still there.
Likely causes
Start here
- Faulty NOx sensor · moderate
- Contaminated oxygen sensor element · moderate
- Exhaust leak upstream of the sensor · moderate
Less common
- Oxygen sensor not correctly seated · moderate
- Engine burning oil into the exhaust · specialist
Symptoms
Components involved
Information, not instruction
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Community experience
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