I have no cabin heat in my 1996 Volvo 850 even though the blower and heater core appear to be working. I have the air mix turned off, so no outside air should be coming in. Nevertheless, the system just blows cold air when I turn the blower on.

850s have what is called a "constant circulation" heater core, which I assume to mean that hot coolant is constantly flowing through the core, so why is the cabin air not getting heated?

(I am aware that sometimes hacks will bypass a broken core by disconnecting them from the core and connecting them together so that no fluid is moving through the core. I have verified that that is not the case for me. Both hoses are attached to their correct ports on the firewall of the engine compartment.)

  • Its possibly a bad switch, or an air bubble in the cooling system. Commented Feb 11, 2019 at 1:34

1 Answer 1


Let's talk about the heater air flow in your system. You are totally correct, there is hot water flowing to the heater core ALL the time. The manufacturers have been doing that since the 1980's because water control valves were hugely unreliable. Instead, they allow you to control heat via air flow thru the heater core. That red/blue control moves a little door (blend door). When it's set to full red, then all the air inside the heating ventilation system is routed thru the heater core. At full blue, no air is routed through the heater core.

HVAC Air Flow diagram

Its pretty obvious, that your temperature control / blend door is stuck on full cool. I'm pretty sure your system uses an electric motor actuator. Older, (and often less expensive cars) use mechanical cables to control the blend door.

Its possible the electric motor is dead, or possible that you've stripped the plastic gears within the door controller.

Blend door actuator

Question for you. With engine off, power on, fan on low blower speed, when you adjust the temperature from full cool to full heat and back again, do you hear any noises coming from behind the instrument panel. If you hear noise, probably a stripped plastic gear. If no noise, dead blend door electric motor.

Depending on your vehicle, this could be a fairly easy repair (perhaps thru the glove box area) or a way difficult repair, involving detaching and dropping the instrument panel. The actuators are normally priced around $25 USD or so.

Side note: You mentioned that your "heater core appears to be working". I'm assuming that you've pinched off the heater core hoses with your fingers when the engine is running and can feel warm coolant running through the hose? If not, its possible there are other factors at play here, related to coolant flow thru the core.

  • That's a great explanation. Is there something wrong with my question that you didn't upvote it? Commented Feb 12, 2019 at 16:10
  • Ships passing in the night? Refresh your page.
    – zipzit
    Commented Feb 12, 2019 at 16:15
  • There are two temp dials (driver and passenger side). If I set to full cool, there is sort of a very slight thunking or tap. If I set to full heat I can't hear anything. In any case I have enough information that I can start taking stuff apart and figuring it out. Thanks. Commented Feb 12, 2019 at 17:40
  • Whoa. A dual system. I'm not positive, but I think that means two blend doors. Not sure exactly how your system is designed, but the odds of two blend door actuators breaking at exactly the same time is zero. Could be a common shaft problem. (Which will require instrument panel removal.) Possible common lead wiring problem? (Any other work done inside the instrument panel, lately? ) Are you sure you have coolant flow thru the heater core? Did you do the heater hose finger pinch off hot flow test? (be safe!)
    – zipzit
    Commented Feb 12, 2019 at 20:08
  • There is heat in the cabin, it is just that it is not heating the air going through the ducts, so it is some kind of venting problem. I am starting to take it apart to figure which doors are not working. Commented Feb 12, 2019 at 22:27

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