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I have a Lennox Furnace Model # GS190-120-140. The unit will not fire up even after replacing the gas vaulve and the limit switch. The unit blower comes on as soon as the thermostate calls for heat. I have confirmed that the ingnition elect, spark is working. Unit may have been altered due to its age. Need a service manual to check if it is wired correctly and or test it.

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Pull off all of your thermostat wires from the control board and make sure you dont have a shorted wire. if the fan still runs you could have a few other problems maybe a stuck relay in the motherboard control try to tap it on the little black boxes.

Posted on Jan 05, 2009

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