cracked weather seal boots. Corrosion gets into the connector contacts and can cause all sorts of ECU error code chasing. When replacing various sensors, not a bad idea to splice in a new connector too --- applies to the injectors as well.
Rear crank seal leaks are also a common issue along with valve cover seal leaks. But you'll need to drop the tranny or pull the engine (for oil pump servicing) to access the rear crank seal.