Furnace blower won't turn on... Forced air gas furnace was working fine for three weeks. Now: Turn themostat up Gas comes on, pilot light obviously working Box heats up for about 90 seconds Humidifier motor comes on (meaning signal being sent by furnace) Blower motor does not come on Gas turns off after a while... probably due to a sensor. BUT: Once while trying to start it... .... .... Box heats up for 90 seconds BLOWER CAME ON... but shut off after about 90 seconds (that's the weird part - what wd make it cut out so fast - thermostat was turned right up) This scenario only happened once but at least it meant that the motor and blower are working. I haven't opened up the furnace yet, but was wondering where I should start troubleshooting this. I can provide a photo of the inside of the furnace, esp any sensor equipment or circuit-boards if required. This furnace is 25 years old but I'm not in a position to replace it right now. Any ideas from some knowledgeable people would help. I'm fairly handy with tools, BTW. Any help is greatly appreciated on this.

Turn of the system at the thermostat. If you turn off the blower, assuming you mean at the tstat, there's no need to turn off the pilot. If blower is running and won't stop, even if turned off at tstat, turn off power by any means necessary, and pilot does not need to be turned off. A "standing pilot" is made to stay lit at all times, running or not.

Check the exhaust port of the furnace. Wasps like to build nests in there and it will not allow the furnace to ignite if blocked. You have a gas detector and it is turned on I assume? I forgot this many times.

ignition module, pressure switch and its tubings attachment orifices plugged

clean ports where gas comes out for pilot light also check gas pressure (gas company)

carrier gas furnaces will try to light three times then will usually go on a lockout for one hour you can bypass the lock out by resetting the power, turn furnace on, remove wires from gas valve, you will need a multi meter to check for voltage to valve, if control board sends a call to the gas valve the valve is bad, if voltage is never present during the lighting cycle your control board is bad

Sounds like the flame sensor. But it could be the thermostat. Is the furnace standing pilot or electronic spark or hot surface ignition. I would start by checking thermostat. Turn power off to furnace take thermostat off wall and take wires marked r & w and connect them together. Turn power back on if furnace works then change out thermostat.Most eclectronic ignition Furnaces have a flam sensor, You could take flame sensor out and clean it up with steel wool or wire brush. If you need anymore help just reply back .

Is this furnace standing pilot or electronic ignition. If standing pilot i would check furnace filter then the thermastat. To test themostat first turn power off to furnace. Take thermostat off wall and take wires labled R & W and connect them together. If you have electronic ignition I would start with cleaning the flame sensor with steel wool or wire brush.If you have and more questions i will be glad to help.

If the burners do not fully ignite, the sensor will shut the furnace down. The lack of sufficient burner flame is a gas valve or gas pressure problem within the valve.
Replacing the gas valve with the original one may get you going. I surely suggest a professional service company for safety reasons and a thorough inspection. A 20 year old furnace is past it's normal life span.

The pilot light will be fine it is gas. The only way you need to relight pilot light is if you turn off the gas. The furnace should restart itself when the power comes back on.

I'm not a furnace technician. But I have worked on several Coleman trailer furnaces. So keep in mind that my terminology might not be exactly correct. But I think I can help you.

I don't know what model you are running, but most of them run pretty much the same. You have a four basic components to check. Thermostat; Ignition Control Module; Pressure Switch; and which ever of the following that you have (Glow plug, Electronic ignition, Pilot Light).

I'm assuming that you are referring to the blower as the "Rat wheel blower" (round wheel looking thing with fins on it). If my assumption is correct and if the power from the breaker box to the furnace junction box (Probably with a on and off switch on top or side), than when you give it power (from breaker box and turn thermostat up) the fan (not blower) comes on. If it doesn't there is a probably a problem with your Thermostat or Ignition Module. If it does come on and your glow plug or you igniter tries to work. Than yes, you are correct, you have to have gas pressure to let the pressure switch know that there is adequate gas supply pressure (gas- 5 to 7 W.C, propane 11 to 13 W.C). The reason for this is if the Pressure Switch doesn't feel the correct pressure. It thinks you have a leak and won't release gas for ignition. After you turn the gas on. If the furnace doesn't ignite. Make sure the Thermostat is definitely wired properly. Now we are down to one of three problems. If the Glow Plug, Electronic Ignition, or pilot light are working properly. Than we are down one of two problems. The Ignition Module or Pressure Switch. Check pressure on the Pressure Switch. If yuou don't smell gas it is probably OK. But it is best if you use a manometer. I have rarely found Pressure Switches bad. Normally if I get to that step. It is the Ignition Module. The Ignition Module kind of like the brain of the furnace. It is located behind one of your front panels. It will have approximately 10 wires going to it. Some will be going to the Thermostat, some to the Glow plug (Igniter), some to the Pressure Switch, and some going to the blower.

I have probably said more than you need to know. If I did I apologize. But to wrap it up. You were probably correct. You definitely need gas for the blower to kick on. The furnace has to light and heat up before the blower will Start. Not to keep rattling, but to check to see if the blower motor will run. Is to switch the Thermostat from "auto" to "run".

Good Luck