I believe a code 13 means that a limit or roll out switch has be activated for more than 3 minutes.
Either the furnace has been overheating/ the flame has been rolling out the front of the heat exchanger/ the limit or rollout switch is faulty. This is code that may be indicating a serious monoxide condition.
Do you have a/c? (dirty coil?) Are all the heating and return vents open or uncovered? Do you have dirty air filters in the heating outlets?
A electrical meter will tell you if either the main limit or the roll out switches have tripped.
A temp rise test will tell you if the furnace is actualy overheating.
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