P225C: NOx Sensor Performance - Signal Biased/Stuck High Bank 1 Sensor 1
What to do
Mention whether the reductant fluid is being used up faster than it used to be. That is the clearest sign of this fault — the system is dosing for pollution that is not there — and it is something only you can report.
Ask for both NOx sensors to be compared before either is replaced. These are expensive parts, and the comparison between them is what identifies which one is lying.
Mention any sharp or ammonia-like smell from the exhaust, which fits over-dosing.
Do not let a catalyst be replaced on the strength of sensor readings alone. These sensors are the evidence the whole SCR system is judged on, so verifying them comes first — that is the single most expensive mistake available here.
What this code means
The upstream NOx sensor is reporting more nitrogen oxide than the engine is actually producing.
A NOx sensor is a more complicated device than an oxygen sensor and its cost reflects that. It works in two stages: a first chamber pumps oxygen out of the sample and measures it, then a second chamber breaks down the NOx that remains and measures the oxygen released by that reaction. The difference is the NOx figure. It needs around 800°C to do this, far hotter than an oxygen sensor, which is why its heater is powerful and why it takes minutes rather than seconds to become useful.
Sensor 1 sits before the SCR catalyst and measures what the engine produced, which is the number the dosing calculation is built on.
So a sensor biased high makes the system over-dose. It injects reductant for NOx that was never there — so consumption rises, unreacted ammonia can pass through the catalyst, and deposits build in the exhaust.
The diagnostic trap is that everything downstream looks fine. The catalyst converts what it is given, the downstream sensor reads low, and efficiency appears excellent while reductant is being wasted.
Rising reductant consumption with no leak and no fault elsewhere is the tell. Compare the two sensors at idle on a warm engine — they should be much closer together than at load, and a bias shows as an offset that does not close.
Likely causes
Start here
- Faulty NOx sensor · moderate
Less common
- Contaminated oxygen sensor element · moderate
- Exhaust leak upstream of the sensor · moderate
- Faulty NOx sensor control module · moderate
- Engine burning oil into the exhaust · specialist
Symptoms
Components involved
Information, not instruction
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