RUUD 90 Achiever blower on no heat
I have a Ruud 90 Achiever, model number RCBA-3765GG17XI, MFD: 2003 using natural gas in my condo. Last week I noticed that the blower was turning on but no heat was coming out. I tried to turning the thermostat to the "off" position and then back to "on". When I did this the pilot would click and the burners would ignite for about 3 seconds and then turn off. After that I took both of the front panels off the unit and nothing looked wrong so I put the panels back. When I did this the blower immediately started and the burners lit and stayed on. My heat was working fine until today. I just repeated taking the doors off and replacing them with the same results. Does anyone know why this is happening?