We just replaced the driver'* side mirror on our GMC Sierra. The previous mirror'* out edge got clipped by a car, only chipping a piece off of the outer casing. After a few months though, cracks developed in the mirror and it started turning dark. I don't recall exactly how long after, but it got pretty bad.

We just bought a new OEM mirror and put it on this weekend. Immediately, the autodimming turned on and we can not figure out how to turn it off. We've researched things on line and tried everything, it will not turn off

* Tried unplugging the mirror and it did nothing, it stayed blue.
* Tried shinning a flashlight on the back of of the interior rear view mirror, nothing, the driver'* side mirror stayed blue.
* Tried covering up the sensor on the mirror part of the inside mirror, nothing, the driver'* side stayed blue.
* Tried holding down the power button on the interior rear view mirror, nothing, the driver'* side stayed blue.
* Tried holding the TEMP button down for the heated outside mirrors, nothing, the driver'* side mirror stayed blue.

Is there anything else we can do? Is there a fuse? Is there another sensor? Should we replace the interior rear view mirror?

Any help would be appreciated!

Does your rear view mirror have a power button on it? I think there is an indicator light on the mirror that comes on when the power is turned on....now in daylight if you cover the sensor , does the rear view mirror change?

The driver'* mirror(not the passenger), works in tandem with the rear view mirror.....so they both should dim with the power on and the sensor blocked in daylight....does this happen to both, one, or neither?

Where did you get the mirror from? GM dealer?

I thought the heated mirrors were controlled by the rear defogger?

You might need a scan tool, that can communicate with the BCM to see if codes are set or read data......

Thank you for the fast reply. Yep, the mirror has a power button on it, we've tried just pushing the button and turning power off and we've also tried holding that down for 10 seconds, the light on the mirror goes out but the driver'* side mirror stays blue.

I just did a test in daylight. While the power was on on the mirror, I put my hand on the back of the inside of the mirror covering the sensor and I shined a flashlight in the mirror. (No change in the driver'* side mirror, it was already blue and stayed blue.) The inside mirror darkened. I then removed my hand from the back of the mirror and turned off the flash light, the inside mirror turned back to normal clear, but no change to the driver'* side mirror at all.

I then turned off the power to the inside mirror and did the same test, covered the back, shined a flashlight in the mirror, it did not darken at all. Again, no changes at all with the driver'* side mirror, it'* still blue.

I bought these right from GM www.gmpartcenter.net.

You think these might be connected to the rear defogger? Is there a way to test that?

Also, do you think there is anything in the inside rear view mirror might be malfunctioning and telling it to darken when it shouldn't? After all, the mirror prior to this was dark too, but we thought that was from being hit.

Thank you again!

Well, obviously, the rearview mirror is working correctly.....

The rear defogger, I believe, only controls the heating element of the mirror....when you power the rear defogger, it automatically powers the heating element in the mirror....

Unfortunately, I have not been able to find a wiring diagram for the mirrors with this option....

So the driver'* side mirror was doing the same thing, BEFORE it was hit?

Like I said before, this circuit may be controlled by the BCM, and a scan tool capable of reading "B" codes/data may be necessary to diagnose.....

So the mirror was ok, before being plugged in? Then went to auto dim when connected? And did not return to normal when disconnected?

Thank you again for the reply, I'm sorry for the delayed response. Yes, this was a brand new mirror and completely clear before putting it on and connecting it. Disconnecting it did not take the blue away, I do think it might be fried already. 🙁

And I think the original mirror went turned color after it was clipped. It was not an immediate issue after getting clipped but developed over a few months. And when it was hit, it only nicked a piece off the casing.
I think i will try to test the BCM like you said.
Thank you.

Just a quick, but important follow up, this is a picture of the mirror just a few days later. I think it does have something to do with the heating of the mirror, do you know what would control this? The plastic is literally melting. (today we just unplugged both mirrors to avoid any kind of fire)

Do you think that BCM would cause this or a voltage regulator?
Thank you again.

Just a bit more information, I think it is an electrical issue. Below is a shot of the new mirror today, this hasn't even been in place a full week and you can see how plastic is being effected. We have unplugged both mirrors now, we are worried about a fire hazard. Would you have an idea what would cause this? Maybe the BCM or a relay or something?

Below is the new mirror, that is blue and below that is the original mirror we replaced. You can see how on that mirror too the plastic started to melt.
Thank you.

Sorry, wish I could be more help, but I just can't seem to find online schematics of the heated mirror....found nothing under a Bulletin search....

Thank you. I am going to take it to the dealer tomorrow. One thing I noticed before we unhooked both, thethe drivdriver'sside was got to the touch when the heater was turned off (the passenger side was cool). When we pushed the button for the heated mirrors, tte light came on for the button and the passenger side started to heat to. Pushed it after a minute or two and the passenger side cooled down. The driver'* side was just constantly hot. Whatever is wrong is frying that mirror. Thank you again. I'll update this post when I find out.

Just a quick follow up, my mechanic found the problem, the sheathing was worn off a few of the wires in in the wiring harness going form the truck cab into the door and the wire for heating element in the mirror was broken. That broken wire was stuck into one of the other wires that had been exposed. That wire must have had juice constantly flowing to it. He'* replaced all of those wires and this fixed the problem. Thank you for all of effort trying to help.