I bet it has a CO327. Encoder sensor.

 

Im pretty sure you need a tech 2 to read those codes, I know mine wont pick it up

 

I'm having the same issue with my 05 av. One time all the lights on the switch were off, the other two times the light on 2wd was on like normal. Is there a way to check the switch and sensor? I'm going to look in the book when I get to work tomorrow. where is the best place to get these parts?

 

My snap-on scanner can read it. The best place to get the encoder sensor is gm. To replace the sensor easily you need to be able to clear the codes. Once the codes are cleared you can put the transfer case into neutral. With the case in neutral you pull the encoder then you take it apart. If the timming marks on the gears arn't lined up mark them yourself. Then you pull the gear over the encoder sensor off, replace the encoder sensor and put it back together. I can do one in about half an hour the shop usually charges 1.5 hours. If you can't put the case in neutral the and you have autotrac you will find it difficult to get the encoder back on, you have to start it on the shaft then twist it to get the bolt holes to line up. The timming marks only line up in nuetral so you must mark the gears yourself before you remove the gear.

 

Do you have the gm part number for the sensor? I just got home from the dealer, they tried to sell me the whole thing.

 

Mine has continued to work without issue for a couple weeks now. I did have a tire that had a slow leak they almost seemed to be related, when the tire got low the 4wd would not work. My tire did not get low enough I did not think to effect the diameter much but maybe a speed sensor didnt like it?
Dave
PS
I am sure it is going to wait till the first big snow to fail me...

 

That is when mine went. First snowfall of the season. On my 04 it was the switch.

 

GREAT
our first snow is tonight...8 inches maybe.
when you said switch do you mean the switch on the dash?
Thanks
Dave

 

apparently a lot of the time it is the switch on the dash. Mine hasn't caused me any troubles in a while....knock on wood

 

I've had the same probs on & off for a while. Had the system check under warranty last yr and they said it was the sensor and all good...like usual dealer needs to work on it 3-4x before they figure it out - I'm guessing its the switch...DrX...how hard is it to replace the switch? Is it the whole 4 button unit or just 1?