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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.