the code would only throw up if there is a faulty injector, injector harness is open or shorted out or there is a poor/low electric current, I would check those for that issue and try fix it as it wont do any good over time. Is it an auto? if so could check the control solenoids and transmission fluid and at a push check the speed sensor as that's what sends the data for the other parts to change gear. only other thing I could think of is something hasn’t been put back correctly or is misaligned after changing the belt.