Air-con does use power, there is a small compressor which powers the system. You will notice this when you switch it on (the revs will drop slightly), but on a car like that i'd have my doubts that it would make a meaninful difference in performance. Even on the 1.0 Daihatsu Sirion we used to have it didn't make that much difference. Regarding the funny smell, that could well be due to needing repaired, be it regassed, cleaned out, or whatever, but i doubt whatever the problem is would use more power than normal.

Thanks for replies I will have a look when I get chance

Hi reference my egr valve problem I put light out and have been using it for a few days and as it was hot had Aircon on and went out and started car today and emissions light back on checked code and only one was p0400 exhaust recirculation flow could having Aircon on be related or is it just coincidence and going to have look at egr valve tomorrow or could it be something else thanks

Hi on the Vectra cdti would it pass not with a straight through exhaust from cat back thanks