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That is a open limit on the unit code. One of the rollout thermostats is open. Some have a push button reset on them and others do not. Check around the burner are for them and behind the valve for a rectangular flat plat with two wires to it, tap on that one with your finger or screwdriver and then try it again. Here is what you are looking for. If any are open meanng NO continuity on a meter that code will appear. Also check the filter after you find the bad one as that can cause the unit to overheat and one to trip. Basically the rollout could be bad or the unit could be overheating.