What is the common reason on 2012 Kia Rio SX not starting?

Well, the first thing you need to do is have your battery checked. Battery voltage could be showing relatively high, but the battery may not have enough capacity to handle a load and may not turn the starter when trying to start the vehicle. Start there, and then proceed through the rest of the starting and charging system; you'll find the concern along the way.

I had the same problem and the engine light wouldnt come on for months so they couldn't diagnose it. It started out not starting ocassionally but then finally the engine light came on and we replaced the crankshaft position sensor. Hope this helps you!