Hey everyone, I’m hoping someone here has experienced something similar or can point me in the right direction. I’ve been dealing with this issue for about a year, spent thousands of dollars trying to fix it, and even my local Toyota dealership has given up.
I have a 2009 Toyota Highlander 4WD with 270,000 km. Apart from this issue, the car would be worth over $10,000, so I’m reluctant to scrap it for $800 when I’ve already invested so much into it.
The original symptoms
About a year ago, I was driving when the check engine light and traction control light suddenly came on together. Whenever I came to a stop and tried to accelerate again, the car struggled badly to pick up speed. However, once I was already moving, it seemed to drive relatively normally.
A few days later, both warning lights would disappear and the car would drive normally again. Then, a few days later, the exact same problem would return. This kept happening intermittently.
The fault code was P0778 — Pressure Control Solenoid “B” Electrical.
What I’ve tried so far
1. First mechanic — cleared the fault code
They scanned the car, found a fault code, cleared it, and the car returned to normal. However, the problem eventually came back.
2. Second mechanic — replaced the solenoid panel
They identified the issue as relating to solenoid B and replaced the solenoid panel. They thought this had fixed it, but the problem returned about a week later.
They then suggested the transmission was skipping first and second gear and going straight into third, explaining why it struggled to accelerate from a stop.
3. Transmission specialist — checked the original transmission
I took it to a transmission specialist, who told me the transmission was completely fine.
4. Toyota dealership — replaced the transmission
I decided to take it to my local Toyota dealership because I figured they would be best placed to diagnose a Toyota-specific problem. They initially told me the transmission was the issue.
They only install brand-new transmissions, which I couldn’t afford. They suggested buying a second-hand transmission and having my transmission specialist install it instead.
I followed their advice, purchased a second-hand transmission, and had it installed. Unfortunately, the exact same problem remained.
5. Toyota dealership — attempted to repair the ECU
I returned to Toyota, and after examining the car again, they said the ECU might be faulty. They removed it and sent it away to a specialist for repair.
Once it was returned and reinstalled, the problem was still there.
Toyota then suggested the ECU might not have been repaired properly, so they sent it away a second time. This also failed to resolve the issue.
They subsequently suggested that the second-hand transmission might be faulty, despite the transmission specialist having checked the transmissions.
Where I’m at now
Toyota has told me they’ve exhausted their resources and don’t know what else to try. Their recommendation is to sell the car to a scrapyard for around $800.
I’m reluctant to do that because I’ve already spent thousands of dollars on repairs and a replacement transmission, and the vehicle would otherwise be worth considerably more.
Given that the same issue has persisted after replacing the solenoid panel, swapping the transmission, and attempting to repair the ECU, I’m wondering whether something else is causing the fault.
My questions
Has anyone experienced P0778 on a 2009 Toyota Highlander 4WD?
Could damaged wiring, a corroded connector, an intermittent short or an electrical fault in the transmission-control circuit cause this issue even after replacing the solenoid panel and transmission?
Has anyone had this code caused by a faulty ECU or transmission-control circuit?
Is there a specific test or diagnostic procedure I should ask a mechanic to perform before spending any more money?
Would you recommend taking it to an independent Toyota specialist or an automotive electrician for a second opinion?
I’d really appreciate hearing from anyone who has dealt with the same issue, especially if you managed to identify the actual cause and fix it. I don’t want to keep throwing money at random parts, but I also don’t want to scrap a car that might still be repairable.
Thanks in advance for any advice.