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If you have a modern furnace, there are fuses on the main board. Check those. Most furnaces have a purge blower that is usually a small motor and fan attached to the burner box that comes on first. This trips a pressure switch once on that turns on the main fan and burners. Both my fan motor and switch have went bad and I've replaced them separately over the past 13 years. If you turn the plastic indicator wheel on the back of the purge fan motor and then it takes off, the bushings are going bad but the motor will run for a few weeks. Also, there's a hi-limit thyristor or snap switch on the furnace. If this goes bad, the furnace will not start.Most furnaces built in the past 15 years have an LED indicator that will blink a trouble code to tell you what's missing or what's wrong. You will need one or two jumper wires to bypass the sensors and a meter to troubleshoot.HTH.Kev
does the blower turn on if you move the fan limit buy hand? if it does fan limit is bad if it doesn't blower is bad.
Pull the wires off of the sensor in question. If the furnace works, it's a bad sensor. If that doesn't work, use a piece of wire and jumper the two terminals. If it still doesn't work, look for the problem elsewhere.
Our furnace doesn't use the fan limit switch to control the blower, it is controlled by the control board. It kicks it on a certain time after main burner fires. The fan limit switch is used to shut down the burner if the furnace overheats.If it is blowing on manual then you can rule out the blower itself.When I was having problems with our furnace last year I got a lot of help from the forums at http://www.doityourself.com/forum/.
Does the blower come on when you turn the thermostat from auto to on? If it comes on it will be the fan limit if it doesn't come on it's most likely the blower motor.
with out seeing it i would call it bad fan limit how many wires are on the fan limit?