York Furnace - blower motor only sometimes turning on
My furnace is about 11 years old and to this point has worked perfectly. This summer the A/C ran fine(I live in VA so it runs a lot) and then when the cold weather came I turned on the heat.
When the thermostat turns on the heat I can hear the furnace start up, the exhaust fan turns on, the gas ignites and it burns for a minute or so but the blower motor never turns on and it shuts off. I then get a code for the limit switch opening...which makes sense because the blower motor doesn't turn on.
If I leave it alone for a while it will cool down after about 5 minutes and try again, and will usually then work correctly. Sometimes it works fine the first try, sometimes it takes 2 or 3.
If on the thermostat I change the fan from "Auto" to "On" the blower motor turns on just fine. I'm a relative novice with HVAC but am learning...Can anyone suggest some diagnostics I can do?