The aux pump can run like crazy and not flow unless the heater valve opens. The aux pump could, indirectly, easily be the cause of the whole thing.

As the aux pump gets old and cruds up, the current draw increases. When it gets too high it will blow the climate control pushbutton unit. This will cause loss of signal to the heater valve. Since the heater valve is normally open, you will get max heat with no mercy.

Start by blocking the flow to the heater valve and see if the heat goes away. If so check for any signal going to the heater valve. If there is none, the CCU likely needs replacing. If you replace it, check aux pump current draw so that it does not blow the new CCU. If the aux pump current draw exceeds 1.3 Amps, replace that aux pump at the same time.

Good luck,