I had a really good conversation with an aircon specialist, so just thought that I'd share to answer this query.....
I had the aircon on my A3 serviced as it was seven years old and although it was still working, it wasn't cooling at well as it used to.
Firstly - if you want to test the performance of your aircon, hold a thermometer in front of the outlet vent with the aircon on full chill (set to "LO"). If you have got one of those "meat probe" temperature sensors, you can stick it into the centre vent to take the temperature of the air as it comes out of the vent.
His basic "standard" is that if the output temperature is 8 degrees or lower, then the system is working to an acceptable standard. If its chilling but doesn't go as cold as 8 degrees (mine was 10) then it definately needs a regas...
After being serviced, the air out of the airvent was down to 5 degrees - much colder.
Out of interest, I took our kitchen meat probe and stuck it into the centre vent of our three year old Yaris - and the temperature went as low as 3 degrees. Yippee - that's still working just fine - and even colder than the just serviced Audi....
So there's your benchmark if you want to test your aircon. Working perfectly, it should be able to chill the the temperature of the air coming out of the air vents down to at least 5 degrees max...)
Secondly (I did know this but share it for anyone who isn't aware) - you should run the air con for at least ten minutes every week or so. When you run it, the oil inside the system circulates and lubricates the rubber pipes and seals. If you don't run it regularly, they dry out and that can lead to premature failure and leaks.
Finally, (and this is the nugget of info that really was new to me) even if the system is still working perfectly, you should change the refrigerant every 3-4 years. (Garages will try to get you to do this every 2 years, he says, but only every 3-4 years is really necessary according to my man...)
The reason you need to do this is preventative. Moisture gets into the system over time - but it doesn't just make the system less efficient, the moisture combines with the refrigerant and it turns it acidic, so that it actually starts to corrode the pipes and seals from the inside.
So you really do need to get a proper service every few years to remove all the contaminated refrigerant and replace it with fresh, to stop the internal corrosion damaging the system.
So that's why, even if the pressure is low, its not worth buying one of the top up DIY cannisters - you need a proper professional vacuum system to extract all the contaminated fluid and replace it with fresh. If you just top up the pressure with a diy can, you've still got the original moisture-ridden refrigerant in your system.
Better to spent £50 on a proper job than £30 on a DIY can....
