TPMS fault warning coming on regularly and 2 visits to Audi dealership and they can’t find any fault. Tyre pressures etc have been checked multiple times and reset but no luck. Any thoughts as to the cause 

Welcome Ron and thanks for joining.

’Tyre pressures have been checked multiple times and reset…. ‘ 

So are you saying that the tyre pressures need resetting -in response to the TPMS warning - so they are actually lower? than the required pressure? 
Regards,

Gareth. 

No, the tyre pressures are always okay when checked. It’s not saying that a specific tyre is low on pressure as it would if there was a puncture etc but that there is a fault with the system. I have found out that it is not sensors in the valves that monitor the tyres but a combination of the rolling circumference when set which is monitored by the speed sensors/ ABS system