Little late to the forum here...
I have a 2007 is250, with code p0751. Hence, I have found a pretty rare TSB: "L-SB-0034-09" that pertains to the specific code p0751, and only affects the year 2007 is250 RWD/AWD. In the TSB, The ECU gets a software update if the code: "p0751" is thrown, and certain parameters on the lexus diagnostics software is met.
However, I would have to pay around $200 for this update. my vehicle would not engage gear 6. and did have the code p0751 when i bought it. If i cleared the code by disconnecting battery it would shift from 1-5, but skip 2nd gear. I manually select 2nd as the desired gear in the S mode, then 2nd gear engages, but takes more RPMS than the other gears to engage it. In limp mode with the code, it engages 2nd gear properly but only for a half second then just goes to 3rd. when i purchased the vehicle it was leaking from the transmission oil pan gasket . I drained and filled the pan and changed the oil pan gasket, changed the filter, and cleaned the Solenoids. However, I still have the issue with no gear 6, increased rpms to accelerate, and when i make a turn it disengages gear. According to the methods to check fluid level I have the right amount of fluid. However i think this method is really bogus and not working because i would drain more than i added, and it would say it was at the right amount when it was clearly lower than before. I did more research and found out that if one changes the pan, or the torque converter there are specific amounts (1.9 q /4.1q) to be added, after the check level bolt is indicating "full". This leads me to believe that possibly I have low fluid. I know the highest gear doesn't engage on automatic transmissions if fluid is low.
I added more fluid, and it stayed engaged in the turns. I tried to go in gear 6, but this time it went into gear 3 and wouldnt change gears. no code, or VSC. I decided to add more fluid, and the car made a strange flapping noise when it turned on. I tried to go in gear 6 and it went into limp mode with a code p0751. Im worried I over filled it. but it feels like its low on fluid still. The person i bought it from tried to fix it when they bought it from a one owner, in 2019. They just drove it around and cleared the battery because they couldnt fix it, i picked it up a month ago.
I think i should add more fluid till i do not have low fluid symptoms, leave the check engine light on, and then have the ECU updated. do not want to overfill it. do you guys think a massive amount slowly drained through the gasket while it was being cleared? I can attach the file that states "fill it up with 4.1 quarts through the side, after changing the torque converter"

 

You should go ask in the IS250 sub-forum, the information/table present here pertains specifically to the LS430 in a sub-forum for the LS430. You'll probably get more help than a "dunno" that way.
@benjamin24