It's a 2004 Trailblazer with 4.2L.

The 4WD light is on the dash. Over the week, I've replaced the dash switch and transfer case motor, but the light is still on.

The engine light has been on for a while for two error codes, and both were for leaks in the EVAP system. One was PO455, but I don't recall the other - one was serious and the other I guess was a standard leak. I replaced the fuel cap last week, so there is currently no engine light. There was never a code for the 4WD.

I've not tried another TCCM. I've found one local, but I've read it has to be programmed. Dealer wants $200 plus fees, so probably around $250 after all is said and done. I've checked all connections and cables, and everything appears to be in order.

What would be the next best step to check?

 

You'll need to scan the vehicle with a professional diagnostic tool to determine the fault that is triggering the 4WD light.
Then post it here and make sure you search the technical bulletins like these:

https://www.tsbsearch.com/GMC/05-04-21-003C
https://www.tsbsearch.com/Chevrolet/04-04-21-004
https://gmtnation.com/forums/threads...89/#post-24787

 

I have a consumer scan tool, and there is currently no check engine lights or faults reported. I presume you are saying that the professional diagnostic tool will provide details that are not showing up on the OBD2 port?

 

I'll post this anyway. We took it out for a drive tonight, and it dawned on me there are no lights showing for 2WD, which is where the switch is pointed. When I initially start up the car, they all light up, but then they all go away.

 

I've been looking into getting codes from the TCCM, and I see they are accessed differently. This will be an interesting challenge, but I program devices through serial equipment, so I'll see how far I get. Thanks

 

Correct. Basic tools are unable to communicate with your Transfer Case Shift Control Module. (it's proprietary stuff, not legislated OBD)
Try swinging by an Autozone, Advance, OReily, etc.