Neither my downstairs heating or air conditioner will come on. The fan works. How do I trouble shoot this?

Do you have the proper electric testing tools and the knowledge in how to use them?

Yes, I have a volt meter and I know how to use it. One other thing. After I asked the question about troubleshooting the system, I decided to take a look at the AC and heating unit. A strange thing happened, my AC decided to work on it's own. I didn't touch anything except turn switch outside for the AC off and then back on. However, it hasn't worked since.

First make sure you have the proper voltage at the unit or proper operation. Then if that is OK start tracing the 24 volt control circuits to find the problem.

See the ''Sequence of operations'' section of the sticky at the top of the forum. Was the switch just a disconnect, or a breaker? Breakers some times look on when they are off. In that case, turning it clear off and back on restores power. If it is not a heat pump, the heat should work without the outside unit.

I've checked for 24 volts at the wires for the thermostat. My meter registered 27 volts. I'm assuming that's OK. The wires I checked were the green, white and yellow, and all said 27 volts.