4Runner Automatic Transmission Shifting Issue

Wow, that sounds frustrating! I had a similar experience with my old Volvo XC60 last year. Same kind of weird shifting and the transmission light was always on. In my case, it turned out to be undervoltage in the system messing with the transmission control unit. It was like the car's brain was getting scrambled messages. Did you check the battery voltage or anything related to the electrical system?

Interesting! I hadn't really considered undervoltage as the main culprit. I did a quick check of the battery, and it seemed okay, but I didn't dig deeper into the electrical system. Was your Volvo showing any other symptoms besides the transmission issues? Could it be that the oil condition is giving incorrect measurements to the ECU?

Besides the transmission going haywire, the Volvo's electrical system would sometimes act strange. The headlights would flicker briefly, and the infotainment system would occasionally reboot itself. I'd suggest getting a proper diagnostic check at a workshop. They can hook it up to a computer and see if there are any fault codes related to undervoltage or the transmission control unit. It's better to be safe than sorry, especially with automatic gearboxes; they are quite sensitive. It sounds like a hydraulic fluid problem.

Thanks a lot for the advice! It really helps to hear from someone who's been through something similar. I took it to a local shop, and you were spot on! It was indeed an undervoltage issue that was causing all the transmission problems. They ran a diagnostic, found some faulty wiring, and after the repair (cost me about 240€), the 4Runner is shifting like a dream again! Really appreciate the help!