My Central Air Won't Turn On

Bob, Can you give us a little more info? Model of unit, is it doing anything at all?

The thermostat must have power from the transformer in the furnace at the R or RC terminal. It should switch power to the G terminal starting the blower, and to the Y terminal turning on the outside unit. When the thermostat calls for heat, the 2 small wires going into the outside unit should have 24 volts AC and the 2 large ones should have 240 volts AC. If so, and it isn't running at all, the relay is bad. An easy fix if you shut the power off first. If not, you need to follow the dead set of wires back to where the problem is.

To do simple checks like this you do need some tools. A test light, a meter, or a voltage detector might be the best place to start with. I came across the niftiest gadget for trouble shooting, a voltage detector. They work through the insulation of wires. There are several brands. I have a GB Instruments GVD-505A, less than $15 at Home Depot. Touch it to a hot wire, and the end glows red. Find the doodad that lights it on one side, and not the other, and you have the culprit. You do not have to open up housings and expose electrical contacts. You are looking at where your hand is, not where the meter is. Most people are capable of doing repairs and will get it going and not get hurt if they use a little sense. The voltage detector makes it even easier.