D4 flashing on the dashboard. What can I do to rectify this?

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D4 flashes on the dashboard of my car when on drive and when this happen it slows down the car acceleration. At times when D4 does not flash, and the car accelerator is applied, it only raises but does not move at the expected speed. The problem expressed above still persist after I have Flushed out the transmission fluid. I later result to change the transmission control. On doing this, I noticed injection light switch on and never switch off, thereby increase fuel consumption. What can I do again.