furnace blower fan and the pre blower fan come on at same time. and the ignitor won't light. have to turn off power switch to stop both fans. i changed upper limit swith but still won't work. when it was working it was going great, returned and blower was running only but no heat. thermostat was set high enough to kick it in but still the fans would blow but no ignition. ignitor is the glass type inspected for cracks but looks good. i just replasced last year.

A roll out limit tripped, look for small disk devices around the burner. They have a small red button between the two wires, gently push the buttons untill it clicks, or check all the limits with an ohm meter to find out which on went bad.