Cause
Contaminated NOx sensor element
A NOx sensor's element coated by soot, oil or crystallised reductant, so it responds slowly or reads incorrectly. These sensors sit in the exhaust downstream of everything, which is precisely where deposits collect.
A coated element responds slowly rather than wrongly, which is the important distinction: the value looks plausible at steady state and lags badly when conditions change. So correlation checks between the upstream and downstream sensors trip while each reading, taken alone, is reasonable — and no static test will show it.
They are expensive, which makes getting this right worth the effort. A contaminated sensor condemns a healthy catalyst if it is believed, and a healthy sensor is condemned by a catalyst fault if it is not. Watch how quickly each sensor responds to a commanded change in dosing rather than comparing their absolute values. Crystallised reductant on a downstream sensor points upstream at an injector or air supply fault that will recontaminate a new sensor.
Typically a moderate job.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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