New SSM from Jaguar, "After reviewing tire pressure monitors returned for warranty replacement we have found that installation of metal valve caps has caused breakage of the valve stems and will not be covered under warranty. Corossion between the metal cap and the aluminum stem is causing seizing. Technicians are advised to remove any metal valve caps and replace with gray plastic caps installed at factory."

If you have metal caps and they don't want to come off try a little heat from a micro sized butane torch being careful of the wheel and damage to the valve stem seal. It doesn't take much heat and has been working for me. I've replaced several tire pressure monitors for this problem.

 

That's a good tip there Real_Tech, I cut mine off with a Dremel. I reckon the torch would have been easier.