Start the engine and allow to idle. While engine is idling, feel for hot air at vents.
CAUTION:
If air discharge remains cool and engine coolant temperature gauge does not move, engine coolant level is low in engine and must be filled. Stop engine, allow to cool and fill cooling system. Start engine and allow to idle until normal operating temperature is reached. Hot air should discharge from vents. Engine coolant temperature gauge should maintain a stabilized reading in middle of "NORMAL" range and upper radiator hose should feel hot to touch.