Four-wheel drive not working?

2003 GMC SIERRA
249,000 MILES • 6.0L • V8 • 4WD • AUTOMATIC
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MLONG98
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Truck is in 2wd now, but
Sometimes my truck would make me use the 4x4 selector switch several times to get it to engage. It would flash between the two options, the one i was on and the one i selected.
Often rolling a few feet would fix this .

After using 4x4 I couldn't get it to disengage for a while. Trying the button many times , turning vehicle off and moving it over n over . Eventually only the rear was spinning , but the service 4x4 light is still on , there was no sign it actually disengaged though.

and now no lights come onto my selector switch except when the keys in the on position " not running " No lights come on my selector switch for 4x4 except when I first turn the key.

Have checked both inside and out 4x4 fuses , Trac I believe it was blown but made no difference when replaced .

So, I've replaced the switch. Checked some fuses. Verified the switch is getting power, and yet my switch doesn't light up or activate any buttons .

There was a small ground underneath the truck mounted to the frame people online said to check. Cleaned that made no difference.

Can't get it to cycle through auto , 2h 4l or 4h.

Another weird thing is I've lost some kind of communication with the OBD port .

My check engine light is no longer on while running , and i know it won't go away without replacing a handful of sensors .

When Obd tool scans truck I'm getting 0 codes every time. Which is impossible .

I've replaced the 4x4 switch on the dash it made no differences. Please help! 2003 GMC Sierra 1500hd 4x4 6.0l gas, automatic.
Aug 12, 2024 at 2:33 AM
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STRAILER
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So, there is a logic module inside the instrument panel that works through the 4wd indicator light which also is connected to the data link connector. I would start by checking the CIG LTR fuse in the under-hood fuse panel for power and check to see if the fuse is good.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Here are the locations of the fuses. Check out the images (below). Let us know how it goes.
Aug 13, 2024 at 4:31 AM
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MLONG98
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Yes, the cig lighter fuse, t rec and the 4x4 fuses are all good. I randomly got back my engine light today after fiddling with all my fuses, found no blown fuses. I will check to see if the obd can tell me anything tomorrow.
Aug 13, 2024 at 12:43 PM
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STRAILER
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I have seen the front differential actuator become weak and cause this issue, here is the location and how to change it out. You might want to remove it and then work the switch to see if it operates. Check out the images (below). Please let us know what happens.
Aug 14, 2024 at 2:21 AM
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MLONG98
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Obd Port working again , after replacing an o2 sensor the only code I have now Is P0327 , knock sensor. It's not related to my 4x4 issue . Fuses are good , obd pulled nothing. what's next?
Aug 14, 2024 at 2:21 AM
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MLONG98
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  • 79 POSTS
I will look into the actuator thing and get back to you.
Aug 14, 2024 at 2:52 AM
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STRAILER
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Let us know what you find.
Aug 14, 2024 at 3:02 AM
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MLONG98
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  • 79 POSTS
Replacing the actuator made no difference.
Sep 22, 2024 at 6:45 AM
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STRAILER
  • Experienced Mechanic
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So does the light come on for the 4x4 and it doesn't work or the light doesnt come on? I would check the power and grounds to the transfer case control module that would be the next item unless it is a wiring problem, here is a guide to help check for power and grounds and the transferase wiring diagrams so you can see which wires to test and the transferase control module location as well.

https://www.2carpros.com/articles/how-to-check-wiring

You might want to just disconnect the module and reconnect it again which can help with a bad connection. Check out the images (below). Please let us know what happens.

Sep 23, 2024 at 2:26 AM
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