2005 Honda Civic. Radiator recently replaced. When I drive, heater seems to blow okay, but when slowing down or idling, the car blows cold air. I checked radiator fluid reservoir and it was empty so I filled it up. Repair shop says I need a new thermostat. is this correct? They found no leaks.
You describe trapped air in the cooling system. To remove any trapped air do this.
Get the engine up to operating temperature. With the engine idling slightly loosen the upper radiator hose clamp. Take a small flat bladed screwdriver and slip this between the upper radiator hose and the radiator hose neck. Allow the engine to idle until a steady stream of coolant comes out of the upper radiator hose. Remove the screwdriver and retighten the hose clamp.
Tester
I think you have an air pocket in the cooling system. Many Honda engines have a valve to allow this air to escape when the system is being filled. Perhaps the air wasn’t properly bled from the cooling system when the radiator was replaced.
Why did the car need a new radiator?
Please check the coolant level inside the radiator (when cold), not just the reservoir. The radiator should completely full, right up to the cap.