We have the same valve. It is the drain valve from our 2 way fridge water supply for the ice maker. If you open the first valve under the sink to provide water for the ice maker and the 2nd valve under the gas range to the the fridge, it is the water supply for the ice maker. For winterizing you have to open that 3rd drain valve on the exterior, as the picture shows, in order to relieve pressure and drain the ice maker. You should not allow anti freeze into the ice maker. Use air pressure to blow it out, then close the valve under the range to prevent antifreeze from entering. Then drain, blow out the rest of the system with air or use antifreeze.
If your system is pressurized, open the 3rd exterior valve, while the valve under the sink is open and pressurized water should flow out.
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2005 Red GMC Duramax 4X4 HD Crew Cab
2018 North Point 315RLTS
4" Magnaflow, Banks Air Intake
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