faults like this can be a pain!
I'd start off by fault code reading the ECU to see if it'll tell you what it's not happy with. Some info here:
https://www.polog40.co.uk/forum/index.php?topic=213.0You'll need a "vagcom" lead with a 2+2 adaptor. There's loads of them on ebay, you'll want a lead for the really old software version (something like 3.11). You might also find that the VWTool.exe program you can download in the above link works better on a G40 than the proper vagcom software.
Hopefully that'll tell you which sensor(s) the ECU isn't happy about. Before you start spending money on new ones, check the wiring from the sensor to the ECU. There's an ECU pin out here:
https://www.polog40.co.uk/forum/index.php?topic=211.0If you have a haynes manual, that's useful too. Just use a multimeter and do a continuity test on the wiring.
If the wiring is OK then try swapping the sensor(s).
I had a similar fault years ago which took me a while to trace, fortunately I had a Polo GT at the time too so could swap sensors between the two rather than just buying new ones. In the end it turned out to be the ECU itself that was faulty. I only found out after trying pretty much everything else and then borrowing a spare ECU from a friend out of desperation.