If the core is bypassed I would suspect it leaks. There are two molded hoses that connect the heater core to the heat transfer tubes on the engine. These are the inlet and outlet positions in order. To replace them you have to drain the coolant and then use clamp pliers to remove the hoses. What I would do before I did that would be to get a pressure test kit and block off one of the tubes and connect to the other, then apply about 15 pounds of pressure and see if the system holds that pressure. If it leaks it will slowly lose pressure.
In that case you will need to replace the heater core. That requires that the dash be removed to get the HVAC case out and apart to get to the core. I would replace both the heat and AC cores while there.
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Saturday, March 27th, 2021 AT 8:32 PM