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Oil reaching the combustion chamber past worn valve stem seals, tired piston rings, or failing turbocharger shaft seals. Its importance in diagnostics is out of proportion to how often it is looked for, because it quietly destroys the parts downstream of it and makes their replacements fail the same way.

Oil poisons oxygen and NOx sensors, coating the element so it responds slowly or stops reading credibly. It fills particulate filters with ash, which no regeneration removes — an engine using oil reaches the end of a filter's ash life far sooner than its mileage suggests. And it coats the inside of a charge air cooler, insulating the fins so the cooler stops cooling.

Which is why it belongs on the list whenever an aftertreatment part has failed early. A sensor lazy well before its service life, or a second particulate filter blocking as fast as the first, is asking what reached it.

Blue smoke is the obvious sign but a late one. Earlier evidence is oil consumption between services with no visible leak, oil inside the intercooler or intake tract, and sooty deposits on the sensor tips themselves. Replacing the poisoned part without addressing the source restarts the same clock.