It would be good if you can use an infrared thermometer to tell if the heater core is getting hot. Is there any place you could get a reading on the core? Are the hoses hot going to the mono valve? If the core isn't getting hot then it isn't likely there is a control side problem.

Monovalve failure causing no heat is common and some aftermarket valves had their share of problems.

If the hoses to the monovalve are hot and the hot water is going through the valve into the core, I'd try CRC ThermoCure flush and give it plenty of time to work. ThermoCure can only be used with pure water, no coolant. The engine must be protected from freezing while using it.

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