Dear j.pelchat,

Hope you're well and stay so. When the climate control system is opened, air enters the system. That can produce an "air lock", a bubble that blocks the flow of coolant through the heater core.

Before you start swapping parts, do the following. Park the car on an incline (a hill), with the hood pointing up-hill. Loosen the cap on the coolant overflow tank (in the engine compartment). Bring the engine to operating temperature. Allow the engine to run for 10 minutes. Then, reverse the car's direction - nose facing down-hill - and do the same. This should allow any trapped air to move to the overflow tank, from which it exits the system.

See if this restores the flow of warm air.

Hope this helps.

Yours faithfully,

Spook