Overdrive trouble.

Hi,i have a celica 2001 gt,recently the overdrive gone,i change the transmission for another one whit less mileage thinking that the problem was the transmission.When i drove whit the new one at the expreesway ,i realize that i have same problem.The overdrive either work.So,i sow a transmission specialist that told me the problem is the engine computer ( ECU).
Some of you know or heard about this? Let me know pleasee....Thank