Hi There,

I have a gmp 1 00-4 Gas furnace. I guess it is more than 10 years old since the house is built in 1995. The furnace was working fine last year, but this year, it stopped working. I have searched the internet and followed some of the instructions and here are the status:

Call for heat, and then there is a small fan started to run, after 20 - 30 s, the hot surface ignition system started to glow, but the main burner never turned on. I checked the power supply to the board and I can see 24v ac goes into the board, but it seems the gv note never produce 24 V to the Gas volve, there are total 4 rollout limit switches betweent the circuit board and the gas valve. I tried to measure the first rollout limit switch out of the circuit board, once the power is on - power indicator on the board is on, there is a ~ 15V at the first rollout limit switch and that power stays all the way until the furnace lock itself down, it never goes up to 24 volts.

Any help is highly appreciated.

Check the string of switches surrounding the burners (follow the purple wire from the gas valve). If center button "pops" when pressed, the switch had opened. This condition may indicate a faulty heat exchanger.