A fast way to tell if the low, or high for that matter, pressure switch on your drying is working is to bypass it. There is really nothing to it and no harm will be done.

First understand that the voltage to the fan passes through the switch on its' way to the fan. There is only one wire and it should be 12v +.

Do the test in this order: Get a voltmeter and find the wiring leading to the pressure switch on the dryer. This is the easy part.

Pull the plug off the top of the switch, start your engine and turn on the A/C. One of the wires leading up to the pressure switch should read 12 v or so. Anywhere between 10.5 and 13.5 is possible, but it should be close to 12 v or above just a bit.

Do you have voltage to one of the wires? If yes then use a small piece of wire, like a paper clip, and jump from one slot to the next on the plug. The fan, if it is working, should come on, and you have just found that the pressure switch is not working.

Too complicated? Take your voltmeter, put it on the ohms setting, pull the plug off and turn on your A/C. If the plug is working you will get a flow of electricity from one pole to another and your meter should show this.

Now you have to decide if you want to just wire in a jumper wire and bypass the pressure switch or if you want to replace the switch which would mean your system would need a recharge.

It could also be that you don't have enough pressure in your system to bring the switch up to contact pressure, but if this were the case your air would not blow cold.

You can also unplug the fan and throw 12 v to it. The brown wire is the negative one. If the fan does not come on you have found your problem.