2008 Civic - vents warm air when set to cold (without AC on)

History: A few months back I had a leak repaired in my cooling system. I noticed the other night that my radiator was nearly empty again, so I’ve filled it back up, but haven’t fixed it yet.

As summer is over, I no longer need my AC. However that caused me to notice that at its coolest setting, the vents are blowing warm air (maybe 15º warmer than outside) and that seems high to me.

My immediate thought is that the heater control valve may be leaking/damaged/corroded.

But I wanted a second opinion from you guys.

Any thoughts?

There might be a problem with what Honda calls the Air Mix Control Motor in the ventilation system

The AMCM is what controls the door in the ventilation system that directs either cold or heated air into the passenger cabin.


If it’s like my 1999 Civic, you can see the heater control valve just in front of the firewall. With the car running, have someone move the Hot-Cold lever or dial through its range. The arm that moves the internals of the valve should move through its range.

You mean you’re feeling warmed air coming through the air vents when you’ve set it for venting just fresh air instead? In years gone by most cars had a water shut-off valve that prevented hot water from the engine getting anywhere near the venting system. That is if you set the temp to “cold”. B/c setting the temp to “cold” physically shut off that valve, so there was no way for engine hot water to get into the system.

But as car buyers wanted more control of the HVAC, a lot of newer car designs have eliminated that valve. Instead hot engine water is always going to the HVAC system no matter how the dials are set, and the design relies on the various vent doors and flaps working perfectly and sealing that hot component completely out when it is set to “cold”. But alas, things working perfectly isn’t such a good idea to rely on. I expect your 2008 doesn’t have that water shut-off valve, and one or more of those vent doors under your dash isn’t opening or shutting 100% like they should.