I recently purchase a 2019 A4 40 TFSi which the Phone Box option fitted.

When I place my phone on the charging mat it charges for about a second then I get a message on the MMI screen saying charging unavailable due to an object detected in the tray! 
 

initially I thought it was the charging mat itself so I bought a second hand on off eBay but when it arrived and I plugged it in I got the same message! I have VCDS and there are no faults stored!

Can anyone shed any light as to what the problem is?

Thanks

Tom

Have you tried a different phone, removed phone case or any thing else attached. When you place a phone on the wireless charger there is a handshake between the phone and the charger which determines if it’s safe to charge and how much power required and also when to stop charging. I’m unsure if this is all done in the charging mat and phone or if the charging mat uses a module in the car to help which could then also be a possible issue. Does the charging mat show up on a system diagnostic scan as I would think that if it does not then it is a self contained device with a basic power supply from the vehicle 

There were no faults stored in the system. Bought a second hand unit and it seems to be working ok, must have been a faulty mat!

So you had to buy two secondhand ones before one of them worked-? 

Yes. First one I bought showed the same fault so I returned it and bought a second from another seller!