Auto HVAC Knock-Knock in rear


Active Member
Several months ago, inspired by a thread in this section, I replaced the rear door actuators as explained in the thread. That thread talked about a click-click sound. At that time, the sound that I was getting was a very loud click that would usually quit after a couple of minutes. It fixed that loud click. Now it does just two knocks at very random times even when the car is not running. Is there a way to get rid of this knock-knock? It almost makes me want say, "Who's there?" Someone has suggested that I need to reset the BCM. How do I do that? Thanks for the help.

PS What are push notifications referred at the bottom of the page?


Maybe an HVAC relearn might help. Pull the HVAC-B fuse, wait about 10 seconds, put it back and turn the key to RUN. Let it do its thing for a few minutes while it relearns the actuators.


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This is the General Fuse Box Layout for BOTH the Front and Rear Fuse Boxes in the 2002-2009 Trailblazers:

I’m not sure that you are going to want to climb into the rear of the EXT Cargo Area on the Passenger Side and tear into all of this stuff to affect these multiple Actuator R&R repairs… but this video will give you a ‘One Over The World View” of all that is involved:

...and here is another Rear HVAC Blend Door Actuator Repair performed with Daylight Views shown in "The Failblazer Project" Part #6:

Last edited:


Active Member
I pulled the cover off of the rear fuse box. There are no numbers printed on the underside. I have in the past pulled the cover on the underhood fuse box and there are numbers there. There are none here in the back. According to the listing provided by MRRSM, the fuse that I want to pull is number 36. Can you show me which one this might be? Or are they the same numbering as underhood? Yes, I did replace the actuators (Stepper motors) as MRRSM mentions.


Well-Known Member
PS What are push notifications referred at the bottom of the page?
These are little pop-ups that alert you to something you want to know that happened on this site. The pushes will show up on your browser even if you aren't on this page whereas you have to be here to see the bell icon at the top right . If you start a thread and want to be notified of responses, it will pop-up, and also change the bell icon with a red circle and a number in it to denote number of items for you to check. If you get quoted, you may (will?) get a notification of that, too, via the push. You would also see it reflected in the bell icon.



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