What are some common issues to look out for when inspecting an old Coleman mobile home furnace?

When inspecting an old Coleman mobile home furnace, common issues to look out for include rust or corrosion on the unit, cracks in the heat exchanger, malfunctioning thermostat, dirty or clogged filters, and unusual noises or smells coming from the furnace. It is important to address these issues promptly to ensure the furnace operates safely and efficiently.