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For you Automotive HVAC guys, 2006 Envoy Defrost only issue automatic controls

My mother-in-law bought this Envoy new and it has proven to be a great vehicle for her. About a year ago her battery went dead and when changed the auto controls for the dash are stuck on defrost. Here is Al that is not very comfortable and she has asked if I can look at it. I have researched that there is a mode control valve that goes through a calibration whenever it looses power i.e. changing the vehicle battery but not at every start-up supposedly. It is supposedly very common for it to stick when it hits one of the stops and then you have the condition of defrost only. Most mechanics on-line are suggesting replacement of this part which is not too expensive. I have not crawled under the dash for this yet but is it possible to reset this mode controller, I’m guessing its a stepper motor, start the learn process and try not to let it get stuck on the stops to finish the calibration instead of replacing the thing? This repair at a shop is easily $275+ and sounds like GM should be slapped for something like this. Maybe it is in the owner’s manual, a correct procedure for replacing the battery but if it is dead when you get up in the morning you aren’t going to be able to follow a proper procedure.

This is the problem:

Looks like you can order a new part and put it in for $42 or you can take the old one out, take it apart, and try to reset it. Looks pretty clear that that’s the problem.

Silly question from somebody who isn’t following you 100%: (That “somebody” would be me.)

If changing the battery caused a calibration problem, would disconnecting/reconnecting the battery reset the calibration enough? …or is there something that is more “stuck?”

Anyone thinking about disconnecting the battery on a modern vehicle should read this first.


@CCarr supposedly this is a design fault that over time the part gains slack maybe. Then when it goes through a calibration, it passes a stop and get completely stuck leaving it there. One of the posts on that blog discussed that a new part would most likely fail at some future time period if the wear is present and the battery was removed again.

@Bing I found Rock Auto has it for $37 but may not get here in time for a trip that she has planned. Will be trying remove and replace with re-cal this evening probably. Hers has hte moving pedals so that makes it a little tougher.

@Tester I agree with most of that website but sometimes you just get a bad day going when you cannot control that the battery died. Think first time drivers… In the case of our Envoy, automotive engineers are supposed to go through failure mode and effect analysis before this module is used to understand all the failure modes including lifetime wear and its effects. Someone hasn’t done their job correctly and allowed subpar designs to be put in customer hands. Unfortunately it is usually out of warranty before the battery needs to be changed the first time and we get the bill for the repair. Which in turn never gets fed back to GM warranty claims and is not put through proper failure analysis to correct the design. More continue to be created the same way on different platforms. The issue propagates.

Some people have had luck disconnecting the battery for 60 seconds with all controls off, though not many, more people have had luck using an obd external power to prevent memory loss, or power plug 9v battery adapter to prevent memory loss and avoid the problem. As this is not a fix for now, advice for the future. Most times for whatever reason it needs replacement.

Yep GM deserves a slap for this one. The problem could be a bad design or it could a manufacturing defect in the actuator. Start with the dealer or go right to a GM factory Rep. See what they say. There is a good chance GM knows about the problem and may have an updated replacement part or may have a modified installation procedure.

My guess for what’s going wrong. During the calibration process the actuator is driven beyond its limit. The controller then “knows” the damper is closed (or open ). The controller then modulates based on that assumption. The actuator is supposed to stall at its limit. Instead, it sounds like the motor drive gear either runs the partial driven gear off the end of its track or the gear teeth are damaged. Either way the actuator is stuck.

IMO rerunning the calibration procedure will just cause the same problem.

From the link you provided, it appears some have had luck disassembling the actuator and resetting the gears, others have just installed a new part. Either should work provided; you never run the recalibration again and the lack of calibration still allows the system to function well enough. After all almost anything is better than 100% defrost.

If you can get an assurance from GM that the problem has been addresses getting a new part from the dealer might be the best way to go. If not repair or replace the one you have and hope you never have to run the recalibration procedure.

Short term fix was implemented this weekend. The actuator would move to full travel and stick every time I tried to cal. Then, a thought came to mind. I left the actuator unplugged and forced the movement to full face airflow. Started the vehicle and ensured Defrost would also blow when commanded and it does. So Mother-in-law is happy that the air blows in her face and can be blocked off with the manual controls. I’m happy the defrost will still blow also. Now we just need to see if this stay put for her trip this week and then I can replace the part when she gets back.

I did take the actuator apart but could not see any of the gears damaged internal or external. I’m sure there is a “procedure” from a manufacturing standpoint on how this stuff lines up but it must be wear that is causing cal to stick it because nothing was absolutely broken.

In the future, when you have questions very specific to the Trailblazer/Envoy/Ascender platform, you may want to cross check over at Trailvoy as the people who frequent that board are very familiar with problems specific to that platform. This has been a known issue for quite some time. Since my TB got crushed, I haven’t spent as much time over there but it’s been very helpful at times.

I usually look at vehicle specific forums but got lazy on this one. I almost never work on this Envoy so I was looking for an easy out if you will. I still think GM goofed on this fault though without an easy solution dollar wise.