VW transponder
See that chip on the end of the key? That’s the transponder that the immobiliser needs to recognise before the car will start. If it isn’t the right chip already programmed into the car, then it won’t start. This particular chip is a VW ID48, sat on a HU66 key.
VW Transponder. ID48 T6
VW ID48 T6 transponder on end of HU66 key






Modern vehicles (since about 2000) are fitted with immobilisers to prevent theft. The immobiliser checks for the correct, matched chip before it will allow the car to start. Some vehicles will just not turn over if the chip isn’t recognised. On other vehicles the engine will turn over but will not start due to the fuel pump being disabled.