The MM codes only show up when the oil life indicator hits 15%. It will say "Maint Due Soon" and will give you the "A12", "B136", etc. codes on that screen. At 50% oil life remaining, you still have at least 2500-3000 miles (depending on your driving style) before you reach 15%. Is there any other indication of any of the other fluids ever being changed? Tranny, rear diff (VTM-4), transfer case gear lube and brake fluid changes are pretty important on these trucks. They're extremely simple to do with just hand tools and a few hours of your time.