D3 is not a service code. It is an indicator that lights-up to the right of the "D" (Drive) indicator on the dash, when you push the button on the end of the shift level. When lit, the transmission will only shift into the lowest 3 gears. It is used when hauling heavy loaded trailers or over large hills/mountains. Also helps with engine braking on steep douwgrades. See owners manual for full details. Look under automatic Transmission.