My furnace (Ruud Silhouette II Gas Furnace) sometimes fails to come on when the thermostat calls for heat. There are two green lights visible in a small viewing window in the lower panel, the right one is solid and the left one blinks 4 times. A soft slap to the top left of the unit starts everything running and it heats until the set temp is reached. It does this several times a day, but not every time. Occasionally it will run perfectly for weeks without a problem, but then reverts to this same problem. It usually will cycle properly a few times before failing and displaying the blinking light again. Can you tell what the problem is from this information?
The 4 flashes are saying that the pressure switch is sticking closed and not releasing when the inducer motor shuts off. When you rap on it the switch releases and goes back to normal operation. If you pull the top cover off you will see a small tube going from the vent motor to the pressure switch. I would pull the tube off and make sure that they are cleaned out. You may have to replace the switch eventually but if you clean it up it may be alright. The door switch on the bottom may not making good contact with the lower door. Pull the doors off and when you put the bottom one on make sure it is securely in place and making good contact with the door switch.