My Tahoe does wierd 4x4 things! Put new 4x4 push buttons in, but no difference. Took it to best mechanic in the area, couldn't find problem, OF COURSE it wouldn't mess up when he had it. Tahoe may work great for a week or mess up all day long at times. SERVICE 4 WHEEL DRIVE comes on sometimes. Actual problem is: sometimes I start up comes up to say auto 4 wheel drive or 4 high or a few times showed neutral or not light up at all and worse yet sometimes it may say it shows 2 high as it should be, but actually it may be 4 low or ???. Many times I can keep pushing the buttons and it will straighten up, but just a few minutes ago: started it up, showed 4 high, I pushed 2 high, 2 high light started flashing & service 4 wheel drive came up and I took off but felt like it was in a bind and in like 4 low with differentials locked in type thing and then I started pushing the buttons and shut off and recranked numerous times, etc., etc.. (that usually fixes it), but this time it never did, I then got a thought to disconnect the ground off of the battery and let it set for 30 minutes, I rehooked battery and cranked and it came up into 4 low, I pushed 2 high and it went in fine???? It's like there is a short circuit in something somewhere??? Also, my tighten fuel cap comes on a lot as well, that same mechanic couldn't figure that one out either and yes my fuel cap is tight, he said the code showed a large amount of vaccum leaking but they couldn't find it, they even added smoke into the system and couldn't see any leaks anywhere. It now has 77000 miles, this started at about 40000???, service 4 wheel drive code started coming on at 60000

 

My best bet would be a faulty Transfer Case Control Module (TCCM).

Depending on the year and DTC code set there may be a new design EVAP Valve that will fix that issue.

 

Where is the TCCM located, is it internal or external, in other words could I replace reasonably easy or is it kind of a major ordeal? Same on the EVAP valve...

 

It is a real pita to change if you never done it before, have a reputable dealer or mechanic complete this the TCCM replacement.

The EVAP valve is easy but I would like to know what DTC code(s) it is setting before I have you possibly waste money on parts.

 

OK, it's just I was hoping I could do it as the mechanic is always about 2 weeks behind. 'll have to ask him again about the code, I had it wrote down but lost it, all I remember is that the code # indicated it was a large volume leak.

 

Also, what is the knocking sound in the steering? I hear it's a common problem? Is it dangerous? Also noticed inside of front left tire wearing (also a little on front right) are these problems related??

 

Not dangerous, the fix is to replace the intermediate steering shaft with updated P/N19153614.