We own a 2004 s80 AWD. It's my wife's daily driver. I love driving it, especially wintertime. She gets the nice car.

Lately it's been in the teens and single digits. Really cold. She says the car never warms up inside. I drove it for about five miles and the dash gauge never got to the center and there was no heat in the passenger compartment. It did get to the quarter mark. But not center.

Our v70, same year, would have centered and had heat in the passenger compartment after five miles. I have a trustworthy mechanic who suggests heater core. I think it sounds like a thermostat. IMHO car is not getting to operating temperature.

The thermostat is part of an assembly. Expensive, for a thermostat. It appears removing the drive belt and moving the steering pump are required for access. Is it difficult?

Does it make sense to replace the heater core. The car has 156k miles. Maintained by dealer according to volvo schedule.
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I'm stuck on Volvo and Volvo's stuck on me....when it comes to FWD I always listen to the "Oracle of Minnesota'

My heater core went catastrophically at the top of a mountain range in the middle of nowhere. Before that though it had shown symptoms, namely that the plastic baffles at the bottom of the core (like a mini radiator) had flecked apart and disintegrated, by making a sound that I had thought was a leaf in the climate blower like blowing around. This noise had gone on two weeks previous to my disastrous trip.
It is very hard to see if it is leaking without pulling apart some of the carpet. Perhaps you can smell the glyc... stuff of the coolant or get fingers under there and feel for moisture.
Alas, when the heater core goes it can really go.

You might also want to check the cabin filter just to make sure the heater core is getting proper airflow.

In crease the temperature on the climate control.

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

Based on the temp. gauge not reaching operating temperature range, I'd also suspect the thermostat. I don't think 11 years on a thermostat is too much to ask.
I suppose it could also be an issue with the heat blending doors in the heating/ventilation system, but I doubt it.

Regardless, I agree with you and doubt it is the heater core.
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"Differences of opinions should be tolerated, but not if they're too different' - Sharon Craig