There isn't a shut-off valve for the heater core. Water flows through the all the time. The only time you get heat is when you set the controls for it and then cabin air is routed through the core. All other times it bypasses it. If you have a leaking heater core, in most cases you can remove the heater hoses at the firewall and using a hose adapter available at any hardware store with a couple clamps, you can make a bypass loop of the core until you can get it replaced. Of course you have no cabin heat until you repair or replace the core. It is not recommended to stub off the ends of the heater hoses because on some vehicles, that loop provides other cooling functions such as cylinder heads, etc. That is why looping it is the safer choice.
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