When I did my SDL last spring I just drained all of the coolant out of the rad, and since I was changing a lot of the hoses the block drained to. I didn't mess with the plug, I just refilled with water ran that for a few days then added coolant. As long as you get most of the coolant, I will do mine every spring I don't want to mess with the block plug.

Btw replace the upper hose at least, it was only $13 last I checked. This is a 603 why chance it?

I think the Prestone says to add with water and then run the car for awhile, that should be good enough.

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2006 CL500
1969 280SE
2023 Ram 1500
2026 Genesis GV70
2007 Tiara 3200