12vdc present CLOSES the valve thus preventing hot water circulation. You can pinch off your heater hose of course but simply inspecting the monovalve is almost as fast and has no potential for damaging the hose.

Up against the firewall on the passenger side. The monovalve has a square top held down by four screws. Easy to see and access. Pull the plug off, unscrew the 4 screws and lift the top off to withdraw the monovalve solenoid. You will lose no coolant if you are parked level. Look for torn/rotted rubber on the diaphragm. Obvious when you pull it out. The complete solenoid assembly is available at a low cost. It may be worth having one on hand before you inspect the existing one. If it hasn't failed yet, it soon will.