Caveman Charlie wrote:
It means that the furnace tried to light 3 times and when it didn't it shut down as a safety feature.
Correct....
Which means main flame didn't light
Could be as simple as dirty connections, dirty gas orifice, dirty burner, dirty spark electrode, spark electrode gap OR failed gas valve, bad spark electrode/wire, bad circuit on control board.
Remove outside access panel
Remove connectors on board...clean contacts on board with pencil eraser and check that pins on connector are making good contact when connector is plugged onto board.
Then try furnace.
If it still doesn't light off you will need to pull burner assembly/spark electrode and clean.
Clean up electrode with 400 grit sandpaper and set gap to 1/8"
Clean/inspect burner.....rust/debris obstructing gas flow
Unscrew orifice and clean with alcohol
Reassemble and test furnace.