There are several possible causes for a furnace blowing cold air, so it’s hard to give you advice without more information.
But here are 3 common causes you can fix yourself.
1) Thermostat is set to ON
Is your furnace blowing cold air only sometimes? Is the furnace’s blower constantly running?
2) Furnace overheated
Let’s say your furnace blows hot air, then cold air, but then stops blowing any air at all after a while. What’s the problem?
3) Pilot light is out
If your gas furnace is only blowing cold air, then the pilot light may be out. Check to see if it is and try these solutions: