The auto headlights/taillights on my 2000 Blazer quit working today. The lights work OK when turned on manually. Is there a fuse or breaker blown, or something else causing the problem.

 

Could be the ambient light sensor in the dash has gone bad.

 

^^agree i replace those all the time on any gm product, also check the relay under the hood put your finger on it see if it clicks, the relay will be labled as DRL

 

The DRL relay does not control the Auto-headlights though, only the daytime running lamps.

 

the daytime running lamps are part of the AUTOHEADLIGHTS realy, they are both controled by the same circut, for all we know he can have a bad relay, or something as simple as a short some where but i know for a fact the DRL relay is part of the auto headlights ive been workin on gm's for 10 years and my expertiese lies in these trucks

 

While they share the same circuit, one is not completely dependent on the other. If the DRL relay were to stick one way or the other, the DRLs would stay on or they would not come on at all. You can test this by simply swapping the DRL relay with another like it in the underhood fuse block. But this would potentially cause his taillights to not come on if the DRL relay were stuck in the energized position. If this were the case, the headlights would be on in their DRL state.

There are a lot of other things that it can be. The logic for the headlight control is attached. The difference between HDLP Mode (which the OP states works perfectly) & Auto Mode lies in the BCM and the inputs it receives from the headlamp switch and, in the case of the headlamp switch being in the Auto mode, the ambient light sensor.

Also attached is the diagnostic chart for an issue with low light automatic activation of the headlamps. A tech2 or equivalent scan tool is necessary to check the BCM I/O's, but...

 

as i do agree with you kyle dont get me wrong, ur a smart, knowlegeble guy like myself. BUT, half of the KIDS on here have no idea what a tech2 is let alone a BCM and what wire to probe to see the voltage, i tryd to go simple, cuz most of the time it is something simple, or opperator error. Like they screwed something up and come on here for there F-ups, but they make it sound like it magicly stoped working. Thats why i said check the relay under the hood it very well can be that simple, or like u said the light sensor on the dash, any who i was just going simple for the guy, voltage, bcm, tcm,ecu is getting out of some guys leauge and will tend to confuse more than help no hard feelings i hope kyle, dont want to make a enemy when sometimes i feel your the only other guy out there that likes to do things i do, DOIN THE JOB RIGHT!!!!!