I have a Lennox G40 series furnace and the gas valve will not open. After it does the preliminary start up I get 27V to the gas valve, but only for about half a second. Periodically the LEDs will state that there is open primary/ secondary limits, but if I jumper those out the problem persists. And by periodically I mean 1 out of 5 times trying to start the furnace from power off to running. The LEDs typically just synchronized fast flash saying there is a call for heat. Does it make sense for the limit switch to be the problem?

  • If you take the thermostat off its base and jumper R to W, does the problem persist? – ThreePhaseEel Oct 1 '19 at 4:27
  • I took the thermostat wires off of the control board in the furnace and jumped R to W. Problem persists. – Ryan Oct 1 '19 at 12:26
  • Sounds like busted control board. – Carl Witthoft Oct 1 '19 at 17:53

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