Driving tonight and I thought the white lines were quite faded. Turns out my lights were not working. A wonder I wasn't stopped by the people in grey.
Pulled into a parking lot, tried changing the relay with a known working one, still no headlights..
Fuses looked ok. Connections to lights looked ok. Too dark to go any further.
Turned the ignition off and the headlights came on for the vehicle exit feature (or whatever it is called).

What controls the headlights through the dashboard switch? Computer issue? Unsure how to troubleshoot this one. No schematic or manual on this vehicle yet.

https://www.alldata.com/diy-us/en

Does it have automatic headlights?

What setting is the **** on?

I can put the selector on manual in the am and the lights come on, but when I start the truck, turn to run, they go off.

I will do more checks based on the switch location after dark tonight to make sure I have correct symptoms. However with the switch in manual they should be on regardless of ambient light or anything else!

The switch was on auto when the lights failed, switching to any other position did not bring them back on.
I'm wondering about any module that the light sensor / light switch feed into.
The circuit for the lights is obviously good, fuses, relays etc. the control of that circuit is the issue. Aren't computers wonderful.

Sounds like the module puked.

Same thing I'm thinking. I am presently 4 hours away from home, work on the road. I don't anticipate any issues getting home as I won't drive it in the dark.
I do have an xtool D7 coming. Guess it'll get its first use in looking at this issue.
Never used a tool as sophisticated as the D7. It'll take me some time to figure out which icon/screen to go to.
This weekend I'll spend some time with it and maybe it will help me.

Other than that, I am assuming this is the BCU but am not sure what modules the truck has or where they are.
I wonder, this is a 2010 sierra and the controls look almost identical to a 2010 DTS, possible they use the same module? (I know where a parts car is)

I've not used that tool. Looks to be similar to a Launch tool. I'm curious to see how it works for you.

I've not used any tool of this caliber. I too wonder how it will work. How detailed a diagnosis it can provide. It won't be at least until this weekend, won't be home until then.

Ok, back home and have the xtool up and running.


xtool is able to control all the lights except the headlights! It correctly tells me that the light switch is active/inactive.
I notice the light sensor reads almost 5v when it is covered but only drops to about 3 volts when there is ambient daylight on it.
There were no codes for BCU or anything for that matter, no codes at all.

Now I am wondering if the light detector is the problem. It doesn't seem to me that a bad detector would keep the headlights off when the switch is set to on and the ignition switch is on. Not sure what putting the ignition into the circuit does to the light circuit.
Not much further than I was.
Not able to set the light detector to low light!

Anyone have any thoughts on how to continue before I wind up blindly changing parts?

I don't like the way this is going. The light sensor would be in a circuit for dimming high beams, it wouldn't be tied in to prevent the lights from working at all.
The light switch appears to be ok by the scanning tool as it recognizes its various positions.
Scan tool cannot turn the lo/hi beams on. Yet with the ignition off the lights do come on.
I believe the most logical item is the BCM but don't know if there is a way to prove it. Then again, maybe the scan tool has done that but not being used to using one such as this I don't realize it.

Any words of wisdom from those here who are more experienced with scan tools?