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Your furnace has several limits so we have to determine which is "open". Lets start with flame rollout limits.They will have a button in center that pushed in to reset.These will be located near burners. We will go from here.I will attach a drawing showing where they are.
Below is a drawing showing rollout and main limit. If rollout is not open check the main limit.To test, you can unplug wires from limits and bypass the limit to see if error code goes away and furnace will work.(Remember only for test.Do not leave a limit jumped)
The rollout looks like this:
The main looks like this:http://www.heatingandcoolingwarehouse.com/High_Limit_Switch_175_F_Part_626424_p/nor%20626424.htm
You can bypass for a test like this:
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