98- LS400 Trans wont reverse
 

1998 LS400 is a very poor thread title.

Flush your transmission oil and use fresh Toyota ATF. I had the same problem with 99 Camry.

after the fluid change did the problem ever come back ?

Have you checked your ATF?

Proper way to check ATF:
1) Warm the car up and/or drive for 15 to 20 miles (with outside temps above 70 degrees or so, if not drive longer/farther)
2) Stop car, shift through each gear and linger in each for 3 seconds or so i.e. 4 wait, 3 wait, 2 wait, L wait, then 2 wait etc., 3, 4, D, N, R, and then park
3) Apply park brake
4) Wait for about 30 seconds
5) Remove dipstick and wipe clean
6) Reinsert dipstick, remove and note fluid level
7) If fluid is low, add small amounts and recheck until proper level is achieved

Any codes set?

I did the same, changed the whole oil with new Toyota ATF but even after the issue continues. won’t go into reverse unless the car is warm.

I had this issue, tried a bunch of stuff. ended up swapping it out with a used transmission