Hello, I have a 2005 Grand Cherokee CRD LTD with a "low spare pressure" warning that will not go away!!!

1. Checked spare, pressure is fine
2. Swapped the spare onto the car which picked it up and gave me a pressure read out correctly, still low spare warning
3. Left the spare at home and went driving, still have the warning

Dealer is trying to make me buy a new sensor but i know there is nothing wrong with it as per the above and they said they can't reset it or anything and keep telling me it will go away after a few drive cycles which it wont!!! Its been 3 months now!

How do I get rid of it? It was only a mild annoyance but now it is starting to irritate me!

Thanks!