Hi everyone,Ive got a 2005 Lexus GS300, 3rd generation, with about 250000 miles, and Ive been dealing with a transmission issue for the past few months.

Once the transmission is fully warmed up, around 100-106 C, and during light driving, I get a P0751 code. Sometimes it appears together with P1201, and in those cases, the car goes into limp mode - it wont shift past 3rd gear. Clearing the code immediately restores normal operation, and it shifts fine again.

For about 1200 miles now, Ive been trying to intentionally reproduce the issue, but Ive never been able to trigger it on demand.

What I do know for sure is:
- It only happens during gentle driving, usually below 45 mph
- It seems related to very light throttle input or coasting
- It only occurs on downshifts, never on upshifts.

Heres what has been done so far:
Repair 1 - Replaced S1 solenoid, plus new fluid and filter - no improvement.
Repair 2 - Replaced the ECU - no improvement.
Repair 3 - Full valve body teardown, installation of a Sonnax Zip Kit, thorough cleaning, replacement of all necessary gaskets, new fluid and filter again. All solenoid wiring and connectors were checked, including the main external connector. Solenoid resistance was measured, and all were cleaned. Adaptations were reset using professional diagnostics. Result - the transmission started shifting like new - genuinely smooth, not just placebo. The error became much rarer. Over about 500 miles and two weeks, Ive only seen P0751 once, and that time it did not trigger limp mode, so no P1201. Before this repair, the error was occurring daily.

At that point, I was almost ready to live with it and just clear the code occasionally, but then something odd happened...Yesterday, my wife drove the car for about 15-20 miles total. During that short drive, the car went into limp mode three times and logged P0751 a total of seven times. I know this because I showed her how to clear the codes...Today, I took the car out myself, pretty frustrated, and drove about 120 miles - highway, heavy traffic, slow, fast, gentle, aggressive - zero errors, zero limp mode.

So there is definitely an issue, but it is extremely inconsistent. My suspicion is that it is somehow related to transmission fluid pressure. Ive also wondered whether it could be the torque converter. One interesting detail - just before the code is triggered, and possibly limp mode, the whole car starts to feel off - kind of like a vibration or hesitation through the entire chassis, but without any fluctuation in RPM. Ive had a bad torque converter before on another car, but the vibration there was more localized, felt under the drivers seat, whereas this feels more like it affects the whole car.There are no harsh shifts, no slipping, at least none that I can feel, and no banging between gears.

All repairs were done using OEM parts and Aisin spec fluid.

Any ideas would be greatly appreciated.

 

My 2006 GS300 AWD used to have a hard shift problem in low speed crawling traffic when shifting between 1st and 2nd. I fixed it 100% by reflowing all the major soldering joints on the PCB inside the ECM.
Not sure this will help you, but when you run out of suspects, might as well just shoot into the dark, at least knowing it will rule out one possible cause. The lack of consistent pattern of the problem in your case appears more like it is electrical than mechanical. Maybe give it a shot with your spare ECM module.
Reflowing the PCB requires good PCB rework skills (that I happen to have) and proper tools. If you are not sure how to do it, have it done by pros. There are ECM rebuild businesses out there. Wish you good luck.
EDIT, if your replacement ECM was a new OEM module, please disregard my post. In that case, perhaps have a good inspection at the wiring and connector contacts.

 

Thanks for the reply, cannato.

Ive spent a lot of time thinking about the possibility that the ECM could actually be the root of the problem. Ive read through a huge number of forum threads about it, including Russian forums as well. The thing is, when an ECM is failing, it usually does not keep triggering only one specific code over and over again. Lets say that both my original ECM and the replacement ECM I bought happen to have the exact same solder joints that need resoldering - what are the chances of that, realistically?

Yesterday, I bought another valve body, complete with all solenoids, from a donor car, mainly to test whether there might still be some kind of residual hydraulic pressure issue inside my current valve body. If the exact same problem remains - specifically P0751 - then I think the next logical step would be replacing the wiring that supplies the solenoids inside the transmission. If that still doesnt help, Ill probably move on to checking or replacing the wiring between the transmission and the ECM as well.

I also have a friend in Varna who owns the exact same GS, but without any transmission problems at all. Im currently waiting for a time when he wont need the car for a day or two, so he can send me his ECM and I can perform one more proper ECM test with a unit that should theoretically be known-good. I dont know when that will happen yet, though, because hes a pretty busy guy.