I've been told our global cooling system is broken?

I've had a engineer look at our global cooling system and he tells us that we've been overusing it and the temperature is sky rocketing cause there's too much exhaust gas or something and as he put it: "luv you've got to fix it", but the estimate is astronomical!

Should I believe him? I'm no good on these things, but as far as I can see its working fine and I wonder whether the thermostat being next to lots of electrical equipment might be affecting the readings. But he says no ... the whole system has to be completely overhauled and we have to got to get in one of these new fangled green heating systems.

What should I do?