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Hello, my truck has heat only coming through the defrost only, I originally thought it may be the control panel, so I ordered a replacement, then under more research I heard it could be the vacuum activated locking solenoid, and was recommended I change it out! Can I have your feed back? Why would this have anything to do with my air flow?
The climate control flow valves in the dash are vacuum operated.
It may not be the solenoid, you could have a vacuum leak.
There is a small plastic vacuum line on the heater flow control valve under the hood at the firewall (on the passenger side). It may have come off, or you may have another leak as Randy mentioned above.