What is cause for check engine light issues with 2012 Nissan Versa w/47700 mi?

What is your question?

Dealer has replaced gas cap, spark plugs and wires, coil, engine cooling fan, purchase gas at different locations, and some sensors. Dealer seems to focus on misfiring related issues as the cause. Have spent over $2000 and the problem resurfaces a day or so after repairs are completed. They kept car 16 days for last repair, fan. Car seems to run well with light on with no notice of misfiring. Can catalytic convertor be checked without replacing? Any thoughts for solution or next logical testing steps rather than trial and error? The car seems to run OK with the check engine light on but I take it for service within two days of light coming on. The light comes on as early as one day after service and as long as a month. I don't know what to add to get this question in an acceptable format to warrant a response from you so I am just going to add frivolous and meaningless dialog to add additional verbiage to fill the box with understanding that quantity is not a substitute for quality.

What makes this problem better or worse?

Goes away following service for a few days but returns.

How long has your 2012 Nissan Versa had this problem?

Several months