Automatic trans will not shift gears at certain times. on 1997 Mitsubishi Eclipse

Transmission shifts fine until check engine light comes on then will not shift into high or low. It stays in the gear that it is in at that time. Might run for a mile or 10 miles before it does it.

Asked by for the 1997 Mitsubishi Eclipse
Sounds like a transmission internal problem but may be cheapest/worth a try to have the transmission and filter serviced and correct Diamond fluid installed to see if it helps.
Thanks. Will give it a try.