My AEM doesn't throw the CEL unless there are problems. My AEM intake uses the stock plastic air intake hose. Now that hose is cracked and a new one costs about 80bucks to my door. My suggestion on AEM, is to get one that connects directly to the throttle body and does away with that plastic accordion stock hose. Here are pics

 

re: used transmissions

How long should I expect my automatic to fucntion properly if I do a lot of easy driving?

 

Until the date of warranty expiration plus one day. Seriously though, there is absolutely no way to predict an answer to your question with any better accuracy.