Sorry, cmcdonnell is right, when there is approx +-12V across the terminals of the monovalve, the valve is actually closed and this is what you should get when you dont expect the heater to be on. This is opposite to what I posted earlier. If you get close to 0V across the terminals, then the pushbutton unit is sending the wrong signal. If you get approx +-12V and you still get heat in the vents, the monovalve is bad, replace it. The rest of the text is still o.k. in the previous post.

The idea cmcdonnell has is a good idea, but when you ground the terminal on the valve, make sure it is the terminal that does not already have 12V, otherwise there will be a big short and you will know it for sure. The pin that receives the signal from the pushbutton unit should be grounded.

Removing power from the coolant pump is much simpler, that should prevent circulation, but if the valve is open, you could still end up getting some heat.

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Saumil S. Patel