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Soot packed into the small passage connecting a sensor to the exhaust stream, so the sensor is sealed off from what it is supposed to measure. The reading it then produces is the most dangerous kind: stable, plausible and no longer connected to reality. Nothing about it looks like a failure, which is why it survives inspection and gets diagnosed as a healthy sensor reporting a genuine condition.

A blocked pressure port is the clearest case. The sensor sits away from the heat with a steel pipe running to the exhaust, and that pipe sooting up leaves the reading frozen near whatever it last saw. Differential pressure across the filter then appears normal regardless of load, and the module either never regenerates or regenerates constantly. A pressure reading that barely moves between idle and load is the signature.

Temperature and particulate sensors fail the same way for a different reason. A soot-coated element responds slowly rather than wrongly, so correlation checks between neighbouring sensors trip while each reading, taken alone, looks reasonable. Clear and inspect the port whenever a sensor in this system is suspected — it costs nothing, and fitting a new sensor into a blocked port reproduces the fault exactly.