I'm about to go pull out the o2 sensor harness and test it with a multimeter, but I noticed part of it plugs into the oil pan, probably the level sensor. Does this plug just pop out or do I have to use a wrench? It doesn't appear to be a hex nut, and it's abnormally large. I tried pulling on the plug a bit and it didn't come off. I don't want to pull too hard if it's not supposed to come off that way, because I don't want to mess up the wire. Any ideas?