Faulty FSR or low AC gas

[X5Kiwi]

I just replaced my aux fan as my aircon was not cold at city speeds and barely cold at highway speeds, and this forum helped me realise that a faulty aux fan was the cause.

Now my aircon is working again, but it only seems to blow cold air from the left side vents. Eventually the right side also blows cold, but it takes 10+ minutes to get cold, and is never as cold as the left side (both sides set to 18 degrees C).

I probably drove around for a month through summer with a dead aux fan without realising, cursing my useless aircon, so I'm guessing this would have put alot of stress on the system.

I've searched the forums, and the two potential problems seem to be either a faulty FSR, or the aircon system is low on gas.

When I set both sides to hot, I get nice hot air as expected, so I don't think it is the temperature sensors in the control panel.

Can anyone suggest any tests to work out the likely culprit?