I've got a 1997 Lennox G20 furnace. A few weeks ago the blower stopped coming on after the burners turned on. The burners would come on for about a minute and then the furnace would shut down because the blower wasn't coming on. Though I could turn the fan on manually from the thermostat by setting the fan to 'On'. The diagnosis was a faulty control board. I replaced the control board with this: https://www.supplyhouse.com/ICM-Controls-ICM289-ICM289-Furnace-Control-Module And it fixed the problem for about a week and everything worked perfectly.
Today, I noticed the temperature on my thermostat was 3 degrees below what it was set to but it said it was heating, but the furnace was not running. I can no longer manually turn the fan on from the thermostat nor does the furnace ever seem to enter its heating cycle when the thermostat clicks on for heat. The furnace doesn't make the clicking sound it always makes prior to the burners coming on, the burners don't come on, it doesn't appear that anything is attempting to work. There is an LED light on a part called a continuous retry that is usually blinking green, but it is not coming on at all anymore.
What might the cause be? I don't see any loose wires, what could I have knocked loose that would be causing the symptoms?
I've got a voltage detector pen, the brown wire coming from the switch to the control board is live, I'm not sure what else I should be testing to see if it's live or not.