2004 Honda Civic heater blows cold air at low speed

Why does my 2004 Honda Civic heater blow cold air when the speed is under 30 mph, or when idling?

Have you checked the coolant level in the radiator ?

Weak water pump ?

That can be caused by air trapped in the cooling system.

When the engine is cold, remove the radiator cap and open the coolant system bleeder screw.

Add a 50/50 mixture of coolant to the radiator until coolant comes out the bleeder screw.

Close the bleeder screw, install the radiator cap and take the vehicle for a drive to see if the heater works properly.