We have a simular problem, we own for many years a RX300 ( year 2000 ), 2WD. Last month the check engine light came on, we own a universal garage so I connected a OBDII scan tool and found code 0171 ( bank 1 lean ) My first thought was : probably a bad oxygen sensor, but I connected a Elmscan interface to my laptop and all voltage readings seemed to be correct. I did reset the OBDII code 0171 and test drove again the car with scantool connected, at that moment the overdrive was working, after a mile the check engine light came on and after 10 seconds the overdrive was gone. I did read the OBDII scantool and again the same 0171. I decided to reset this code again, continued my testdriving and yes at that moment the overdrive was working, but after one or two miles check engine light came on and overdrive not working anymore. I have been reading on several forums and know I'm not the only one with overdrive problems. I think the problem is not inside the transmission, we did oil changes / oil filter every 30k miles. Could this problem be related to the knock sensors ? Can anyone give me a logical explanation for this ? Andy / Dutch Caribbean.