When I turn the thermostat up, fan comes on but not burner. After a few seconds the burner turns on but goes off after about 3 to 5 seconds. The green light on the right is on all of the time. Second green light blinks and the yellow light starts to flash when the igniter starts, but the burner will only come on for a few seconds then will cycle 2 or 3 times, but won't stay on. What's the problem?