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I have an Amana Air Command 90 furnace that doesn't light. When I turn the thermostat on, the fan comes on but the burners don't light. I have a diagnostic light on it but it only comes on for a second when the furnace is turned on and then goes out - it doesn't flash. About to call a tech but it isn't dire yet, mid-50's outside. So I have a little time. Help!
M Oglesby :
Which fan is coming on?
Is it the inducer fan coming on or the blower motor coming on?
The large fan at the bottom of the furnace.
It sounds like you have an open limit switch. There are two kinds of limit switches, the main limit and the manual reset rollout limit switches. We will first try and locate the main limit switch. It is usually located inside the furnace cabinet on the back panel wall behind the gas valve. It is usually rectangular about 2 inches by 3 inches and has two wires attached to it. Locate it and remove the two wires and jumper them together bypassing the limit switch. Attempt to start the furnace with the wires attached together and let me know if it comes on or not.
I located the main limit switch right behind the gas valve and jumpered the wire together and turned the furnace on - it didn't light but the main blower came on.
Place the wires back on the main limit switch. Now locate the manual reset rollout limit switches. They are small round metal disc looking devices about the size of a nickel that have two wires attached to them and a little button on the back of them. They are usually located around the burner tray. Locate them and make sure the little buttons are pressed in. Remember that there may be more than one so look carefully. Once you make sure all of the little buttons are pressed in, attempt to turn the heat on again. Let me know if the heat comes on or not.
Do you happen to see the igniter glowing in the burner chamber? If so then does the unit show a bit of flame before it fails? If you don't see the glow, the igniter is bad. If you see the glow and a bit of flame, then the flame detector has failed. This is a small rod near the igniter and sometimes can be cleaned by using sandpaper to clean off the oxidation that accumulates over time.