A red tag means your equipment is unsafe.
Warning tags are issued on any appliance or piping system that uses natural gas when a licensed gas technician has identified a hazardous issue.If carbon monoxide is leaking from a cracked heat exchanger, there is a chance of it mixing with the air that is generated by the furnace and pushed through the vents in your home.
To prevent this danger, a red tag is applied to your furnace, your local utility is notified, and your natural gas is turned off.