Hmmm....I am willing to bet that the "SPARE" is the one not hooked to a hub, regardless of which of the five rims you have....

IE: Only the hubs connect the tire to the system, so the spare isn't on a hub...and isn't monitored. (When you rotate, the tire that goes from a hub to the spare position becomes the next unmonitored tire, etc...)



Easy way to tell, look at your spare next to one of the known working rims...the rims have a channel for this purpose...., the spare rim and the normally mounted rim should both have this feature.

Example of the other way: My wife's Mini Cooper has a little airline connection you screw onto the spare tire's valve stem before storage...the car monitors the spare's pressure via that connection.