I have a small Mobile home type furnace. The flame comes on but the fan doesn't At 1st it was iintermittent Then it got worse (this was all yesterday.
Under the fan is a box with a removable cover. On top there is a fan switch. Inside there is a transformer with 4 wires and 2 little quarter sized thingy's with 2 wires each. Sometimes I can hit that box with my fist and the fan comes on. All the wires seem to be free. The furnace had the gas controller replaced last year and they also replaced the battery powered thermostat.
Thanks for your help in advance.

I forgot to add the furnace is at least 15 years old and I did see a model # on it.

Also The fan turns off long before the house reaches the temp setting on thermostat.

The fan control isn't working properly. There are several types, here's a few common ones...

Well today is a little different story The fan has been kicking on but I have to keep the thermostat set about 10 degree over the temp wanted. Although when I took these photos the furnace was very hot and the fan was not running. Here are a couple photos of the box I described. Im guessing one is the fan control.http://i133.photobucket.com/albums/q63/sidmabb/Furnace/boxbest.jpg
http://i133.photobucket.com/albums/q63/sidmabb/Furnace/boxtop.jpg

Here is the new controller.
http://i133.photobucket.com/albums/q63/sidmabb/Furnace/controller.jpg

The thermal disc switch with the orange and blue wires connected is the fan control.

Thanks a bunch. Now comes the task of finding one.

Fan switch should be here in the morning but I think there is something else wrong. It quit completely to night so I pulled one of the wires and hooked them together. With the fan and flame running it still puts out very little heat. I looked in at the burners and the flame just looks wimpy. I think it is not allowing enough gas to get through. I wonder if there is something plugged. Does that flex line have a screen that may have corroded shut or something? I see there is some sort of adjustment on the controller. The one in the picture that looks like a tire valve. There is a little threaded rod under that plastic cap.
Its getting cold out so I hope I figure it out.

If the furnace is LP-fired, you may have a problem with the supply regulator. It is less likely that the gas control has become out-of-adjustment.

Gas company just left. They replaced the outside regulator. He said he thought the old one was OK but changed it out to be safe. It didn't help. I think its time to call a repair man. He will be able to test things.
Thanks for your help.

Fusnace man just left. Cob-webs inside the furnace was his opinion. I have heat
Thanks again

This is very plausible. In fact, I'm relieved to know that I'm not the only one who EVER encounters this problem. The eight-legged freaks spin cocoons inside the gas orifice(s) and don't think anything of it.