frank (Goodman Forced Air Unit)

I have a goodman force air unit that the fan comes on at the time the thermostat is turned on and the pilot will not lite once in a while the flame will come on and go out but the blower will not shut off I disconnected the thermostat and the fan comes on when it feels like it'and runs until it is unplug. Have any idea what is happings?

 

does this furnace have a hot surface igniter or is it a standing pilot like your water heater more than likely is. turn the furnace switch off, check to be sure that the burners are clean and clear or any obstruction and be sure all the safety switches have closed before trying to turn it on again. there also may be a flame sensing rod in front of one of the burners that may be dirty and will need to be removed and cleaned with steel wool. check all that first as well as the obvious: clean air filter, gas valve on and so forth and be sure the fan is in the auto position and not the on position at the tstat.