Wire the valve so you can run it yourself LOL. Dam dude this is weird! I don't know if this makes a difference, but did you ever replace the ENTIRE monovalve with a new one? It's just a thought, but if you've only replaced the plunger assembly then the electrical connections are still old. Maybe there's a chance something's wrong with that part of it?

I know my heat used to do weird stuff (weak at idle, random full heat only, random no heat, too cold for the setting I chose, etc.) I did an overhaul on the heater side of the climate control to include: the aux. water pump, the monovalve, the overhead temp sensor, and the pushbutton unit itself. I did all of this with BRAND NEW PARTS. More costly, yes, but I'm one of those people that doesn't do used parts. I like fresh stuff, especially when it comes to plastic, rubber, and electronics/electrical, because I'm convinced that stuff gets lazy and brittle over the years. Let me tell you, it works like a champ now!

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C