There are different reasons why your furnace is blowing cold air. The first thing you should do is check if the thermostat's fan setting is 'on.' If the fan setting is 'on,' the blower will continuously circulate air even if the furnace isn't heating. Turn the fan to 'auto' instead to stop the blower. Secondly, check the filter for any potential blockage. If the airflow is blocked, it can cause overheating in the furnace. Make sure to turn off the furnace to check the filter. Then, remove the dirty filter and replace it with a new one. Contact us if you need any help with your furnace.