Cause
Deposits or scale on the sensor probe
Scale, salt or mineral deposits built up on a sensor's probe, insulating it from what it measures. It is the fluid equivalent of soot on an exhaust sensor — the element is intact and no longer in contact with the medium.
The reading becomes slow rather than wrong, which is the important distinction: a coated probe tracks changes late, so it reads plausibly at steady state and lags whenever conditions move. Correlation checks against a second sensor trip while each value alone looks reasonable, and no static test will show it.
What deposits says something about the fluid. Scale in a coolant circuit means the wrong coolant, water topped up too often, or a system long overdue; crystallised deposits on a reductant probe mean fluid drying in place. Inspect the probe rather than testing the circuit — the coating is visible and conclusive. Cleaning may recover it, but the deposit returns unless the fluid or the interval that produced it changes.
Typically an easy fix. Often worth checking before paying anyone to look.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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