check engine light on 94 v6?

I have a check engine light on my camry. My mechanic hooked up his obd2 computer to it and it pulled 2 codes. First was an 02 sensor,so I bought the sensor and replaced it. Light still does not go out? The other code is for a solenoid "e" in the transmission. The o/d light or overdrive light is flashing telling me about this problem. Does the check engine light also come on for the solenoid for a transmission problem? I took the terminal off the battery and the check engine light will go out. It stays out for as long as I decide to drive the car 2 hours or 2 days but when I turn off the engine and restart it the light is there? Ive done this repeatedly and it is always the same. Why doesnt it come on the first time while Im driving? Mechanic suspects sludge and told me to run a cleaner in the tranny for awhile before we go into it? Thanks in advance for the help!!! :)Nelson