Air Con isn't working....again

Goudy

You need to find a AC specialist.
Your friendly auto centres like Halfords tend to just regas and take money from you, at a push they might tell you you have a fault but they won't identify what/where.

The first thing they should do is measure the gas pressure inside the system and depending on what it currently is will depend on the next action.

If it's low or non existent they will recover the old gas.
This involves attaching their machine to the system and putting the system under vacuum to recover the gas and boil off any moisture in there.

If the system holds a vacuum there is no leak and it's safe to refill.

If it won't hold a vacuum, there's a leak and the system needs checking.
They do this by using a dye, filling the system with nitrogen gas and using a light to try and spot where the leak is.

Leaks can be small and the vacuum test might not show up a leak unless it's left under vacuum for a while. It also might take a bit of dismantling of parts around the engine bay to get a good look at the system.

If it's holding pressure than there is a different type of fault with the system.

The system works on pressure, turning gas to liquid and back again and if it thinks the gas pressure is low the AC compressor pump will not turn on. (the pump has a electro magnetic clutch on the pulley, this is what you usually hear clunk when you turn the AC on).

A pressure switch will inhibit the system if it detects the pressure is too low by stopping the magnetic clutch engaging, but the pressure switch or wiring could be faulty so the pump stays inhibited.

To help with the gas/liquid changes, it will utilise a fan on the front of the A/C condenser, very much like the fan on the engines coolant radiator.
Again, if the fan isn't working it will stop the compressor pump for operating so the AC magnetic clutch won't engage.

As it regassed and worked for a while then stopping again I would suspect a leak.
The gas pipes are all connected with seals that can perish, special lubricate adding to the system helps with seal life but they do go brittle over time and the condenser on the system is up front in the line of all the grit and stones which can lead to holes.
Also the condenser can corrode in places with age, so you might be looking as some parts and more labour to fix.

While parts are stripped off the system (or it's been leaking for a while) it will not doubt require a new dryer as moisture will enter the system.
This dryer is a small canister filled with charcoal to help trap the moisture in the system.