My car has AC but it is not as cold as the AC in my friend's car

This is usually where I come in and at this point I start asking questions. Many of the answers I would really like to know as they may require specialist knowledge but there are some questions you could ask yourself and make some judgements upon.
1. Bearing in mind the rule of thumb in the section above, how long since it was last charged? If you have owned the car for less than about two years and have no other evidence then look further.
2. Is there enough air coming through the face-level-vents? Turn the blower right up, put the air control to face-level and feel how much air is coming out, insufficient could be the result of blocked pollen/dust filters if you have them or a dislodged piece of air trunking under the dash or a vent control that has stuck or even a blower motor not pulling it's weight. Try pushing the recirculation button (your handbook will show you the logo for this) - if more air comes out of the face-level vents then suspect the pollen filters.
3. The heater may be turned off but is it still working a little and spoiling the AC? Turn off both heater and AC and with the car running on tickover for say ten minutes, the bonnet closed and the blower operating, check that the air from the vents is only at ambient temperature and is not heated.
4. Is the AC compressor actually running at all? Look under the bonnet at where the drive belt is running and get a friend to switch the AC on and off a few times, not too quickly as some systems do not start for several seconds after the switch has been pushed. You should hear a distinct click as the clutch operates and you may then be able to identify which is the compressor, you should then be able to observe that when the AC is turned off the belt turns the pulley only but when it is switched on, the outer plate (which is connected to the compressor internals) turns also. If it does not turn, look first at the fuses - it could be something really simple. Should an AC fuse have blown, before you replace it, check that any electro fan on the radiator is able to turn freely - if it is stiff it may draw excessive current and blow the fuse again. If no fuse has blown probably the most likely scenario is that the gas pressure is so low that the safety switch has operated to prevent the compressor turning and risking damage. Double-check that the blower is actually running - if this fails the AC will not run. Early systems using the large black cylindrical Harrison or Frigidaire A6 compressor (mostly Jaguars and Rolls in the UK) frequently used a strange three-legged fuse mounted on the front end of the compressor, under adverse conditions this blows to protect the compressor and to ensure that the system is looked at and preferably recharged before the fuse is replaced - this is one fuse that almost certainly means that a recharge is necessary, don't be tempted to just replace the fuse without having the system checked out first.
5. Once you have ascertained that the compressor is running you could try following the path of the thin pipe coming from the bottom of the condenser (the front radiator). This will often go into a type of metal bottle (the receiver/dryer) that will frequently have a small glass eye in the top of it (the sight-glass). Occasionally this sight-glass is independent and is set into a separate small block. Clean the glass and observe it whilst the compressor is running. Is it virtually completely clear? If you can see a continuous stream of bubbles or even what looks like foam then almost certainly the system needs recharging. It is perfectly normal to see a few bubbles each time the compressor starts and stops and if you have the later R134a refrigerant a few bubbles may be seen continuously on a very cold day but otherwise the sight-glass should show only a continuous flow of liquid. The majority of Vauxhalls and Volvos do not have a sight-glass as most of them use a slightly different system, as do some recent Fords.

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