When I am traveling at 55 or above, my vehicle will not downshift when acceleration is required. When I am accelerating from a stop, the car seems to take off normally through first and second gear, but then goes into seventh gear, causing slow acceleration. In addition, when driving within the city, from light to light, the vehicle accelerates and shifts normally.
It sounds like the transmission valve body has something sticking in it, not allowing downshifts in higher gears / speeds. I would start with a transmission fluid service to rule out low / poor quality fluid. Then move onto computer diagnostics. Your vehicle is equipped with complex control systems. A trained technician, like one from YourMechanic can read the computers and resolve your shifting problems.
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