Now that you know the sail switch is back in position, you should make sure that the switch part is actually making or breaking the circuit. With no power to the furnace
, put an ohm-meter across the switch contacts and manually trigger the switch. It should go from open to short (Zero Ohms), or vice versa.
Next is to re-connect all the push-on connectors. Notoriously known to corrode and/or make bad connections. Pull each one off with pliers, then push back on, checking the wires for good looking crimps (or solder).
Then with power on, make sure that there are voltages actually getting to the circuit board.
After that, You are on your own. The high voltage igniter circuits are sensitive to moisture and short circuits to ground. The sparker tip is sensitive to positioning, but if you are not hearing it trying to start then that's where you have to start. Thermostats go bad - short across the terminals and see if that works, bypassing it. If it starts but doesn't stay lit, or goes out, the low voltage millivolt circuits are very intolerant of bad connections or open grounds. And the thermocouple has to be in the hot part of the pilot light
, if you have one of those.
In my case my fridge
required a new board and I was able to get a replacement made by Dinosaur for it, with improved capabilities. It was just over $100 - they probably also make ones for furnaces.
A lot of what I am advising are just general troubleshooting tips, not specific to your furnace
, BUT a systematic trial and error and process of elimination can often solve these pesky problems.