Sixby2

Hi all , my name is Chris and I run a small garage that mostly deals with diesel, I recently bought a 2002 2.5 v6 auto x type ,I love it, but have a code read from souls p1780 , if I clear code, car drives in 4 gears not selecting 5 th. If I have red box light illuminated I get 5 gear changes with reduced power . Now if I start car and select "d"quickly I get no red lamp and car has full power but only changes 4 gears . I hope somone can help .....