It coulde be coolant level, check that first. It could also be the actuator (either electric or vacuum) that is causing the problem. Check for any vacuum leaks. Does it have climate control?jasrking said:The heater on my 04 silverado works for awhile then gets cold..if i shut the fan off for a few minutes and turn it back on sometimes it will be hot again but other times it will still blow cold air.....thermostat perhaps????