1. Check fluid level 2. Internal malfunction

1999 Chevy blazer LS 4.3 shifting great...but have a code that says 1 of 2 things ..1. Check fluid level 2. Internal malfunction. I have checked fluid level and is normal....what else could it be?

Shifting great just getting that code....can't figure it out,and i can't get it inspected cause of it...