The three LPG cars I've owned (and I still own two). Two of them are different makes of system.

But they ALL have a little square black plastic box on the dash. They all have but the one button on this box. If you push it then it disables the LPG and your running on Petrol (though I have no idea why you'd want to do that- given it's more expensive than liquid platinum).

If you want to more permanently disable the system then again all three of my vehicles, installed by different dealers at different times have another thing in common. Under the bonnet is an in line blade fuse holder and fuse. Inside a waterproof casing on a line from the positive battery terminal to the ECU box. From memory I think it's five amp (or so) fuse. Simply pull the fuse.

Why are you seeking to isolate what to my mind is a very desirable thing to have on a car these days? If you end up having the system removed do put it on Ebay. There is a voracious market for LPG kit especially as with the ever sinking pound and the depression we're in, the price of petrol is going to soar in the near future. Then anyone running on LPG is going to be smiling broadly.

We live in Germany and want to go back home for a visit on the ferry . Thay won't let the car on with out isolating the gas The gas was fitted when we got the car.My German is not that good to go to the dealers and ask the questions. But thanks for the help.