I have a York gas furnace that is about 18 years old. The fan is blowing continuously. Tried to replace the thermostat and it didn't correct the problems. I am assuming the fan limit control switch is bad. Can I replace it myself? It is a Honeywell switch. Should I remove it and take it with me to a supply house?

Does the limit switch control the fan directly, or just the coil of a relay? Any relay could be stuck to. The older furnaces are much easier for a DIY to trouble shoot. If it is a limit switch or relay, likely you can easily take it to a HVAC supply and match it up, or at least go home with a work alike relay.

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.