Transmission shift problem?

P0731 †Gear 1 Incorrect ratio

Fault codes never say to replace parts. They only indicate the circuit or system that needs further diagnosis. In this case a gear ratio error means the wheels aren't turning as fast as expected compared to the engine speed. That's a result of slippage in one of the clutch packs. To verify if the clutch plates are worn out your mechanic can use a scanner that displays live data to view the "clutch volume index", (CVI). That is a set of four numbers that correspond to the volume of fluid it takes to apply each clutch pack. Based on those numbers an experienced transmission mechanic can determine how much life is left in each one.