Hi. I have a ducane unit heater dula045n and it will not shut down.The green light is flashing 4 times. The ignition control LED codes says for 4 flashes means high limit or rollout switch open.How do I fix this problem

If it's a roll out limit switch that is open you have to manually reset the limit. On most units you depress a small button on top of the limit--- located top of the burners small round disck with two wires going to it. On some you kill power to reset. If you reset the switch and it trips again do not reset!! You could have major problems. Its not unusal for a roll out limit to trip during high winds.