How do I measure the fuel trim %? and I guess if my o2 sensors weren't cycling properly, they would eventually throw a check engine light?

 

Eventually they throw a code when something melts but they can be 100% shot and still work for years. You either need an OBD2 scanner that shows realtime data or use an Android smartphone with the app Torque Pro (or even the free version I think) and a Bluetooth OBD2 scanner that will interface with the phone. Preferrable I use a BAFX Bluetooth OBD2 adapter, works great.

Not sure what options exist for Apple phones.

 

I have a scangauge II that I use all the time, although i dont think its in real time. It can read check engine light codes and clear them though. I understand about the o2 sensors being bad, but still working and not throwing a code though, that makes sense. and unfortunately i use ALL apple products and iphones.