I'm trying to replace the spark plugs of my 2006 Prius, but the deep well 5/8 socket attached to a 2 in extender just turns loose, as if it does not fit in the top of the spark plug. I tried also 3/4 and 13/16 but those don't even fit in the spark plug well. I tried in another spark plug to the same result. What could I do to remove the spark plug?
A dedicated spark plug socket is the way to go here.
Regular deep well sockets don't work because the spark plug electrode will interfere with the square drive end. This is also why an extension will not help; there is nowhere for the electrode to be accommodated.