Toyota Mark X 2007 suddenly goes in "limpmode", when in R or N it goes Forward.

There could be a bunch of things causing this but I think the most likely is that you are having a problem with one of the clutch/band pairs. And, it’s not that it is failing, so much, that it may just be due to a problem with the transmission fluid level or flow.

Your first move, then, is to let your car heat up for 20 minutes or so and then pull out the transmission dipstick. Now, reinsert it and draw it out again. The level should between the HOT or H lines (there are two, bottom and top). If the fluid level is okay, you know, at least, there’s enough transmission fluid in the case.

However, even though there may be enough fluid in the case, it is quite possible that the transmission filter could be blocking up and cutting the flow of transmission fluid through the case to a trickle. If it is a trickle, there isn’t enough fluid likely getting up to the clutch/band levels and if that’s the case, then the result is just as you describe it.

Of course, it is possible that the torque converter is not working correctly and is failing to set an adequate flow so that the proper gears are activated. Again, the result is limp-home mode.

You can easily check the fluid level yourself. I’d suggest taking it to an independent service area to have it checked in more depth as they have the proper facilities to drain the fluid and lift off the transmission cover without drowning themselves in fluid. I’d bet dollars to doughnuts that you’ll find that the transmission filter is clogged and messing things up.