Hi,

I have Pacifica 2005. When I drive it automatically, it does not run at more than 60 km/hr, and I think it uses maximum the third gear. However, when I strat drive it manually and then switch to automatic, it works and runs as fast as I want, and in this case, it uses the fourth gear. is it a problem of computer?

 

It has gone into limp mode. Take it to a competent transmission repair shop. There are many possible causes. It could be the computer or it could be something mechanical. No point in guessing over the internet.