Responses are going to be tempered by the fact that we are getting only one side of the dispute.
It would be helpful if you were very specific regarding the product, the problem, and the other trade or service involved. Otherwise we may give you an answer that you will use to justify your position and we could be completely off track.
In general; if I install something it works before I leave the premises.
If I sell you a product with installation, and that installation requires another trade, then I either perform that function myself, or tell you when I sell it and you agree to get the other work done, or it is my responsibility to get it done.
For example, if I were installing a new gas water heater with a power vent (I don't do that sort of thing but it is an example that requires multiple trades.) where one had not been installed before, I would expect to be responsible for the whole job, soup to nuts. When I left, you would have a reliable water heater with the gas, electricity, and plumbing hooked up. The unit would be fired up and I would verify that it was working and controlling.
If I couldn't do the gas line or electrical, I would tell you and we would agree ahead of time who would get it done and who would pay for it.
Now if I were replacing an existing unit and it just involved connecting the new where the old was, I would expect that you would be responsible for the capacity of the gas line. If you need a bigger line from the meter that would be your responsibility unless we contracted for it.