Quick update,

So I ran Economy air this morning after getting under the hood last night. I disconnected/reconnected the monovalve, and wiggled some fuses/cleaned connectors. Perfect operation this morning with no heat on MIN setting. Now, of course, I'm remaining cautiously optimistic, since I'm pretty sure my car is home to a family of gremlins, and I fully expect the problem to resurface again later.

@sleepstar, I replaced the temperature sensor several years back after the previous one crumbled in my hand (messing with center vent vacuum pod), but it's possible this could be playing a role. The quality on available parts for these cars varies so widely that it's not unreasonable to think it needs to be replaced again.

My thought process would be to test the monovalve for failure points again. Might need to buy a new repair kit, but I'll take any small win I can get.

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1985 300D
1987 300SDL
1970 Jaguar E-Type OTS
2002 Dodge Intrepid (Traded In)
2011 Ford Crown Victoria