Why does my 2004 sierra heater only blows hot air?

If your 2004 Sierra's heater only blows hot air, it could be due to a malfunctioning blend door actuator, which controls the mixing of hot and cold air. Another possibility is a stuck or faulty thermostat that prevents the engine from reaching the proper operating temperature. Additionally, low coolant levels or a clogged heater core may also contribute to the issue. Checking these components can help identify and resolve the problem.