GM-GMC: daylight running lights, ambient light sensor, headlamp switch

Question
I have a Chev 2005 Uplander and have noticed just recently that my daylight running lights don't come on. I've checked the headlamps and the low beams, high beams and turn signals all work. I have the lights set to Auto, which is where it has been set since I've had the van. What could be the problem?

Answer
Without being smart or sarcastic I need to ask how it is you know that your DRL's are not working.  The reason I ask that is, most DRL systems do not work until the vehicle is put in gear.  So, if your van was running and not in gear and you approached it, it is possible that the DRL's would appear inoperative.  
   That being said let me explain the system quickly, an ambient light sensor is used to monitor outside lighting conditions. The ambient light sensor provides a voltage signal that will vary between 0.2 and 4.9 volts depending on outside lighting conditions. The body control module (BCM) monitors the ambient light sensor signal circuit to determine if outside lighting conditions are correct for either daytime running lights (DRL) or automatic lamp control (ALC) when the headlamp switch is in the AUTO position. In daylight conditions the BCM will command the DRLs ON. During low light conditions the BCM will command the headlamps ON. Any function or condition that turns on the headlamps will cancel DRL operation.

Because all of these functions are controlled through the BCM body control moudle, it is entirely possible that the BCM is faulty and causing your DRL's to malfunction.

It is a fairly simple system, but they can be difficult to diagnose.

Hope this helps.