2007 Hummer H3, automatic tranny, 5 cylinder. I can put vehicle into 4hi lock & back into 4hi..I can also hear the electric motor on trans. Going into & out of 4lock, but, when in park, neutral or let vehicle roll forward a bit @ a slow pace & push the lowlock button repeatedly id does nothing..the button wont even light up & I dont hear anything from the trans.. so..Guys, any ideas?
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Sounds like the button isn't working. If the button was working the LED would flash when you pressed it while it tried to shift to 4 low lock.

 

First read the owner's manual.
Second, clean the T Case buttons with CRC Electronics Cleaner spray...... let that dry.

Then try it.

 

X2 on what Doc said.
Could be the button for 4HI unlocked is stuck and won't release.

 

Try repeatedly pushing the button.

Sometimes its enough to clear carbon off dirty contacts. To clear problem.