i have a 97 silverado, heater blows cold air ??

 

Make sure you have enough coolant. Let the truck warm up and feel the coolant hoses that go into the bulkhead by the passenger side. They should be hot. If not, your heater core is either plugged up with debris, your hoses are bypassed or collapsed, or your heater box has a stuck door, or your blend door cable is malfunctioning.

Good luck