Q: decelerating engine while approaching highway speed.

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When coming to highway speeqd 45 to 50 , pressing strongly on the accelerator when the transmission should be shifting into high it Wil not shift unless you let off accelerator , if you continue to press firmly on accelerator and engine deaccelerates dramatically and will not shift unless you let up on the accelerator pedal .

My car has 40000 miles.
My car has an automatic transmission.

Failure of the transmission speed sensor is a likely cause of this behavior. You have remarkably few miles for this model year! Replacing the speed sensor may be a good time to change the transmission fluid and filter as well - simply because of age and hot/cold weather cycles. A certified professional from YourMechanic can assist you with a diagnosis of the shifting problem and can replace the transmission speed sensor if this verified as the problem.

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