Does anyone know how the TPM works on the XE ?
So far as I am aware, with the runflats on my BMW, the system measure the rotational speed (which is directly proportional to the rolling diameter and hence the tyre pressure) of each wheel and compares the four wheels tyres, generating an alarm if one tyre is rotating faster that the others. In 8 years I've had the alarm twice, one a genuine puncture, the other a false alarm. Resetting the system is simple enough and it does seem to work.
On the other hand, I believe that more modern systems use a transponder in the tyre which sends an RF signal to a processing unit in the car although quite how the transponder and its associated electronics is powered reliably for the life of a tyre is beyond me. Also, this assumes that the slimline spare (and any winter tyres) have the same transponder system.
Oddly enough, I've not had a convincing answer from any dealer that I've asked. Not a particularly urgent problem, but I do like to know how these things work !
Ian