1 . While the blower is running, check for 12v between the white and yellow wires at the ignition control board.
If you're not getting ~12v between white and yellow wires at the control board, then check for 12v between one of the yellows and each contact of the sail switch, then each contact of the limit switch. One of those two items, or the connection to them, (or maybe at the relay, if the relay side of the sail switch isn't getting 12v) are bad.
2. If you're getting voltage between white and yellow at the control board, then check between the yellow and red wire at the control board. (Begin testing as soon as the blower turns on, and keep testing for a while. There's probably a delay between the time the control board gets power, and then begins to emit, and then another delay when it shuts down after failing to detect flame. Sorry, I don't remember off the top of my head what the delay, if any, should be.)
If you're not getting voltage between the red and yellow, but are between while and yellow, then there's probably an issue with the ignition control board.
If it is the Ignition Control Board, I used the "Dinosaur Electronics FAN50PLUS Universal Igniter Board
" to replace the one in my old furnace. Slightly more complex to install
, but turns off the blower motor on older furnaces without a fan controller if the furnace fails to ignite, saving from heavy, continuous, battery draw.