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Engine oil wicking along a wire into a connector body, where it films the terminals and adds resistance that no visual inspection reveals. The oil travels inside the insulation rather than over it, drawn by capillary action from a leaking rocker cover gasket, a failed sensor seal or crankcase breather mist — so the connector can look clean and dry while the contact inside it is coated.

That makes it a persistent misdiagnosis. The resistance it adds is small and temperature-dependent, so it produces intermittent and range/performance faults rather than clean failures, and it survives the replacement of the component it was blamed on because the new part plugs into the same oily connector.

Fix the source, not just the connector. Cleaning the terminals without repairing the leak buys weeks. Where the oil has travelled far enough into the harness, the affected section has to be replaced — a wire full of oil cannot be cleaned in place.