Is there a call for heat?
Go to thermostat and make a call.
Check fault codes in furnace and follow to rectify. Printed on the door of furnace.
Always read and write down code on service order before shutting power off or opening blower door so code is not deleted.
Is combustion blower running?
If not, make sure there is a call for heat. Check fuse on ignition board, check 120v. power to unit.
Is ignitor glowing?
If not, check fault code. Could be blocked condensate lines. Check to see if ignition board is bad. Meaning, no indicator lights blinking with power supply on.
If ignitor is glowing, gas valves should open and burners light.
Do burners go out after 5 seconds?
If so, clean flame sensor with sandpaper. Remove all tarnish and reinstall. Try again.
If burners stay lit, does blower come on after 30 seconds or so?
If not, check for bad blower motor or capacitor.