Fan comes on NO HEAT changed the Mercury switch still not heating. Pilot is on
What to do now?

Need to know brand of furnace

day and night unit
approx. 33 years old = model 24/80 80 vac 106c

Sounds like you may have 2 different problems. Most older furnaces the fan was controlled by a limit switch. It shouldn't come on until the furnace warmed up. Go to the sticky at https://www.askmehelpdesk.com/heatin...ons-58313.html It I mostly about newer furnaces. You should be able to find a picture of a gas valve. If you had a volt meter, you could check to see if the gas valve had 24 volts AC on it. If so, and the gas isn't on, the valve is bad. If you lack experience, it might be better to let a service man change it.

If it doesn't have voltage, it could be a simple wiring problem. See what you can find and post back.

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.

Had PG&E out
Paid for service call they told me my mercury switch is out they don't carry that part anymore get a new unit
Found the part by chance at appliance shop installed and still no work
Today took back mercury switch part will be in tomorrow morning might have been defective part

You can bypass the switch by connecting the wires together. The furnace should come on and run as long as the wires are connected. Red and white if you have more than 2. If the furnace doesn't run that way, you have other problems.