I'll second what both responders have said, The full info on the codes would be needed to give you a starting place to diagnose any possible problem.
The 700/900 FAQ section you can link to off the main page should give you all the info on how to retrieve the code and how to reset the OBD computer (erase the code) If it was indeed a false code it won't return.
Replacing a computer because it registered a code sounds like BS to me also but, if it did need to be replaced you can find them used for ALOT less than new or remanufactured. It's not a high risk part to get used as they rarely fail. They are also pretty simple to replace, a first-timer with basic tools should be able to do it in 30 min.

My guess is your mechanic has determined that by lightening your wallet your car will run better.