Lexus ls430 p0778

[Damiane619]

Hi I’m experiencing a problem with me 06 ls430 I just recently got the engine swapped due to overheat and it has a p0778 code it shifts only up to third gear and the downshifts are really harsh has anyone experienced this and what can I do ? Or where should I start?

[RX400h]

What is P0778 LEXUS code meaning? The Pressure Control (PC) Solenoid Valve an electronic pressure regulator that controls transmission line pressure based on current flow through its coil windings. As current flow is increased, the magnetic field produced by the coil moves the solenoid’s plunger further away from the exhaust port.

[Boris T]

I had the same problem with the same code. After doing hours of research and reading other posts, I would like to help others if they ever see this post. 

Before I write about my experience, I own a Lexus LS430 2005 in pearl white. Beautiful machine inside and out. When I first experienced a hard downshift, I had around 140k miles on the car which seems to be the area where this problem has occurred with other LS430 owners. 

My father and I, thinking it was a faulty solenoid valve, lifted the car and replaced it (solenoid B I believe, after looking at the transmission schematics). We paid $250 for an OEM solenoid valve from Toyota and replaced it, changed the transmission fluid and nothing happened. The car still did a harsh downshift. The original solenoid was fine as well after doing a voltage test on it. We were frustrated to say the least and we had no plans on going to a dealership to get charged an arm and a leg with no guarantee of a solution.

My original theory was that it was the ECM (we owned a Lexus LS400 as well with other EMC problems). Sure enough, after doing more reading, many others have commented on this issue. The ECM was damaged or defective. It was a known issue as well. Based on recommendations from other Lexus LS430 owners, I shipped my ECM module to SIA Electronics in Chicago (they have experience with repairing ECM units with the same issue). They repaired it for around $150 and shipped it back within about 3 days. We installed the ECM unit and car ran fine ever since. 

I did this solely based on others recommending to repair the ECM first. My father did not listen and chose to work on the transmission solenoids first. Of course there may be real issues with the transmission or solenoid valves, but in our case the transmission and solenoids were fine. 

I HIGHLY recommend anyone who owns an LS 430 to not give up or move to an expensive option such as rebuilding the transmission until you have the ECM looked at. You will save SO much money, time and grief if the ECM turns out to be the culprit. The ECM repair and installation is a VERY easy process, and you will save money by doing it yourself. Don't go to a dealer or shop unless you absolutely have to. 

We repaired the car back in November 2021 and today is June 23. My Lexus has been running fine ever since with no codes, harsh downshifts, or anything. You don't have to purchase any new ECM units. Just repair the one you have and see if that works. If not, explore other options. 

I also plan on purchasing a backup ECM unit programmed to my model just to have for an emergency because I want to put at least another 150-200k miles on it. I REFUSE to purchase a newer Lexus model, and I will put in as much work and maintenance as possible to keep it going.

I sincerely hope I can save someone time and money with our experience, and I hope no one will ever have to go through this. You live and learn. 

[William Boyne]

Hello Boris,

Yes, very informative, thank you. I have a 2003 LS 430 And I have electronics issues. Such as the care won't start. Whats a good resource for trouble shooting this issue, step by step.

Thanks again,

 

Bill Boyne, Houston TX,77095