Furnace flames cycles off before temp is reached

First thing to do is make sure the output vents aren’t shut off. Some owners will close off vents to force more air out other vents. Open any vents closed or otherwise blocked. Check the return for blockage too. These furnaces need good airflow in and out to keep from overheating. I’ve read questions on versions sites about needing a filter on the return like us done on home units. While you would think so, the answer is no. So check if previous owner put in a filter on the return.