I have a 1999 S70 that has lukewarm heat. The temp gauge is normal so I had a local garage look at it. He checked the coolant and that was OK. He thinks it may be one of the vacuum operated doors. I called the local Volvo dealer and they believe the problem is either the thermostat, the heater sensor or the doors. Any thoughts before I turn the car over to the dealer?

 

I would replace the thermostat first then see how it is.