EDIT: Which engine do you have?

Coolant flow through the heater core is constant, there is no valve to regulate the temperature. Unless there is a leak in the core, I wouldn't bother replacing the core.

I agree, the thermostat has failed. Luckily, the design that Volvo uses leaves the thermostat open rather than closed - which will kill the engine rapidly.

Yes, the thermostat and ECT sensor are now combined in a single unit. There is one person on the Brickboard who replaced the thermostat, the ECT sensor is part of the housing and cannot be replaced separately.

See if you can do a search for the replacement, otherwise you will have to replace the unit at a substantial cost.

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My name is Klaus and I am a V♂lv♂holic