Four wheel drive problems 1999 chevy suburban on 1999 Chevrolet Suburban 1500

I have replaced about everything that has to do with the four wheel drive on the suburban and it still want engage I don't know what I'm missing need help

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2 answers 2 comments
Scan test.
I put a scanner on it and nothing came up. The service four wheel drive light is on but no codes.
Did you select the propper test? Code reader or a SCANNER? If the light is on there are codes.
agree with wetry but at this point you may want your mech to ast you in diag and poss repair