I took my 93 Previa to a transmission shop because my car doesn’t always upshift soon enough. I was told that the transmission was fine but that I should replace my thermostat. I was told that the temperature sensor controls everything that happens in my car and it isn’t shifting up because the thermostat is not registering the correct temperature. Does that make sense?
I don’t know if it is true for your car, but I understand it is true for some cars.