Red code lights flash "43" in sight window and furnace won't turn on (Lennox CX34-18/24B-6F-2) Has a Honeywell TH832OU1008 thermostat. What's the problem?

You need to open the lower panel on the furnace and find the code legend. This legend will tell you basically why the furnace won't light. I'm not sure but I think code 43 on lennox furnace, is "Open limit. The furnace has several limits, check to see if the button on them has popped out, if so reset them, also the high limiot which is located on the wall of the furnace. You can check this limit by pulling the wires off and jumping them together, if heater works you need to replace the high limit, be sure to replace it with the same temperature rating