When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
Hit it harder with the impact. There is a sealant there, it might be gluing the nut to the cover. Worst case is the threads are galled and no matter what you do you'll strip out the threads on the cover.. Not much can be done if that's the case.
Well bugger - I posted too late - I'll leave the posted response below if only for reference.
The threads on these are designed to be interference fit, they're also tapered, they are also in two disimilar metals and may have sealant (although it shouldn't be needed - somebody could have used it), none of this is going to help especially if the previous spanner monkey was actually a gorilla.
If you have galvanic corrosion in there it will be a bear to break free, much better to use impact than a long bar but you need an impact that's up to the job - that isn't trying to be defeatist or critical of your gear. You need to get heat in there which will be hard to do with the sensor / wire in the way, heat it, hit the plug with freeze spray, then hit with penetrant rinse repeat a few times - but keep in mind that this plug is designed to keep oil in so penetrant may be of little use. You then need to rattle the plug as quickly as possible after a heat/freeze cycle - you are trying to heat the cover NOT the plug.