Lets start by explaining the error codes and then how the furnace cycles. Steady light is normal, 0 flash means control failure - possible bad control board, 1 flash system lock out - 3 tries and locks out for 1 hour and tried again. 2 flashed means the air switch is stuck closed while 3 flashes means the air switch is stuck open. 4 flashed is high limit - no air flow or low air flow, flame detected with gas valve closed. There are more, but these are the common ones. On call for heat, the control board checks and makes sure the air switch is open. Then the venter motor comes on and the switch must close. Failure gives the above list #2 & #3. Once that is proven we start the hot surface igniter and 1-3 seconds or so later the gas valve opens. Ignition starts and we look for feed back from a flame rod. If the flame rod does not see the flame, the unit locks out and tries 2 more times. If it failed 2 more times, error code 7 flashes. So lets look at the causes of air switch issues - There is a water drain trap in the furnace. All 90%+ furnaces create a lot of water. This collects in the secondary heat exchanger which looks like a radiator just above the main fan and below the primary heat exchanger. If water is unable to leave the heat exchanger, the air switch will not make properly. Tapping on it may be just enough to make the switch. Pull off the hoses going to the trap and the heat exchanger and blow them out with pressure from your mouth. NEVER blow into the ports of the air switch as you will damage it. This resolves about 80% of all furnace issues. If you had a manometer, you could measure the pressure to the switch which would confirm the above.A reading around .2" of water column would be normal. Good luck.
