How old is your battery? This ECU is extremely dependent on a good voltage at turn on which can cause different problems intermittently. Your disconnect battery resets the ecu. Lexus and transmission shops for years did not diagnose the LS430 battery quality and transmission issues. I personally know this along with 2 others. I even know of one that traded in there car because they didn't know it was the battery and nobody could figure it out.

 

Early 20s eh?
I got comfort that the previous owners were 50s/60s ha

 

Thinking the same thing now. I just got home of driving over 30 miles straight on the freeway today, and no shifting problems popped up whatsoever. No CEL either. The gears were holding fine too with no slipping.

 

The battery is new, it was replaced this past October with an Interstate, correct part # as well.

 

Yup, I know, I should have known better 😩☹️😥

 

I would check the ECU first. It’s the cheapest option and a well known failure point. As mentioned before, weak batteries can manifest symptoms in odd ways. Check it out as well. Do the cheap and easy ways first and get a solid base to diagnose the rest of the issues. If you decide to look for a used trans, try to find one with a known VIN number associated with it. Input that VIN into the Lexus owners site and see if that trans has been serviced at a dealer before. Use the car (that the trans came from) MX history to help make your decision as to which one you choose to install in your car.

 

My recent experience. I was having hard downshift issues with my transmission. Codes pointed to replacing the shift solenoid valve. I replaced it along with about 5 to 6 quarts of fluid and it lessened but did not solve the problem. Toyota, Lexus, and a trusted independent shop techs all suggested to replace the transmission. Based on information from this forum, I discovered the ECU issue and sent it out for repair. My friendly suggestion is that you do the same. I agree with the others, spend the small dollars to have the ECU refurbished before you spend the big dollars for a new-to-you transmission.

The transmission now shifts beautifully, a big improvement. In hindsight, the shift quality had been deteriorating for the last few years. Fingers crossed, the smooth operation of the transmission will continue.

 

Thank you guys for the advice. I plan on sending out the ECU this coming weekend.

I found another oddity yesterday while driving the car, if I put the gear selector in S mode, and leave the gear range in 5th or 6th gear without me upshifting manually, the transmission will automatically upshift through all the gears without it getting stuck in 3rd.

But if I go back into D, the transmission will act up and get stuck again.

So it has to be a computer or electrical related problem. Again, the transmission shifts very smoothly, it just gets stuck in either 3rd or 4th gears, so I am hoping that the ECU fix will finally cure it.

If not, I will start sourcing transmissions for replacement.