The 1st thing I do is go to the thermostat and put the Fan switch from Auto to the On position. If the fan operates this proves you have both high and low voltages and the breaker, fuses, transformer etc is functional, this saves one from going to the breakers and air handler often in an attic or roof or basement.
If the fan does not operate then I check for correct power.
Set the thermostat for heat mode and turn the temperature setting well above the rooms temperature. and observe the furnaces sequence of operation.
What you normally expect is upon the white circuit from the thermostat being energized with 24 volts, the induced draft motor will cycle for 30-60 seconds typically to pre purge the heat exchanger, after this purge the induced draft motor has a pressure switch that senses the induced draft operation thus proving the induced draft motors running and alloqw the ignition to begin, either a spark or a HSI, hot surface ignition type.
If you see no bright white light or hear a sparking noise by the burner area this is normal, this will be followed by a trial period for ignition, meaning the gas valve will be energized for a 30 second trial period, during this time the flame rectification proves a flame exists by a DC signal being sent back to the module by the flame sensor or the ground/burner ground.
This DC signal is very weak and easily interrupted and this is why we clean flame sensors and simply re seating wiring will often times correct a situation,
The typical issues are the induced drat pressure switch and the orifice that it's tubing connects to on the induced draft housing.
This port must be reamed to clear carbon and if not will cause a problem such as locking out requiring resetting power or a 5 hour wait for some units,
You need to insure the limits and roll outs are all closed, its all failrly simple if the LED is tellingyou the error as they do today.