how to fix air condition with 40 low pressure and 100 high pressure?
Could be a couple of things. Does the ac compressor turn on at all? Does the system just have a low charge? If the compressor is running and only making 40 psi on the low side then it could be the veins in the compressor are weak hence going bad and you would need a new compressor to which extent you would want a new receiver dryer, orifice tube filter if fitted with one, and expansion valve. Then draw a vacuum for at least 30 minutes or more then recharge the system to what ever the ac tag states on the vehicle.
Check your pressure switch,may be bad.