I have a Ducane 80% gas furnace. It will work OK for several days or a couple of weeks, then it will click on, the igniter will light, but the burners will not light, until I turn off the thermostat, let the blower stop,then turn on the thermostat and furnace will turn on and work ok. Or when the burners won't light , if I go to the furnace and take off the main cover and tap on the gas pipe going to the control valve the burners will light. This has been happing for several months now. A couple of days ago I took out the three burners and cleaned them up, they were not too dirty,and I put it back together and it worked for a couple of days, but it is still not starting up some times.I also took off the clear hose to the pressure switch and blew in that to clear out any dirt. The furnace is about 10 years old.
Anyone have any ideas what could be wrong with it?