The sensors on these cars if they are the original beru ones last for about 6 years.
If any one of the 4 road wheel sensors gets to the point of not working reliably the system will give an error in the dis and there will be no readings in the mmi screen.
The system can indeed monitor the spare wheel but it does not need to have one for the others to work as intended.
If you have four new sensors fitted you need to select the setting in mmi to start the process of learning and go for a long drive. it took the best part of a 30 mile drive for my new sensors to kick in and start working.
Its worth remembering to make sure you don't have spare sensors or old ones in the car during the learning process as this may confuse the system.
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