I have a 2004 envoy and my air will not blow through my front vents. The air blows fine through my top, bottom, rear and side vents, why is it not blowing through my front vents?

The knocking is probably the blend door actuators. They have been detached or become stuck. They operate the doors that direct the flow of air.

https://www.google.com/search?q=2004+gmc+envoy+blend+door+actuator+location

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During the thermostat replacement your mechanic probably had to disconnect the battery. When reconnecting the battery, the HVAC system may not have recalibrated itself properly, or one (or more) of the doors (or its actuators) may have become stuck. Try the following; place all the HVAC switches and controls to their OFF or CCW positions, disconnect the battery for about a minute, then reconnect the battery. Do not touch any of the HVAC controls for at least 2 -3 minutes. If this procedure does not resolve the vent issue, you will need to have the HVAC system diagnosed.

Could be a blown fuse. That's most likely. If not, it may be the switch for the front.

I just changed my battery as well in my 04 Envoy XUV and the air on the passenger side would not blow. The air has since returned after I restarted but the flow on the bottom has been out for some time even before the battery change.

Check and replace the right side temp-controller.

Is it a new vehicle? I had a new vehicle that had the same problem. Answer was that the holes weren't punched out of the vent.

the resister block needs changing... oddly enough i'm unsure of what fan you are saying is not working.... if it's the front.. the resister block is located on the duct work under the dash for the air... near the fan... you will see a connector going into the side of the duct work. two screws and the block comes out... should be fairly obvious for the rear one but it will be near the fan.

Try replacing the cabin air filter.

You most likely have a bad mode actuator. http://forums.trailvoy.com/showthread.php?t=40235