My engine on my 2014 Corolla started to sputter and I took it to a mechanic who replaced the #2 ignition coil and all the spark plugs. A week later the engine started to act up again, but only after it had been running for a while 15-20 minutes on hot summer days. If I left it for a few hours it would run fine again initially for another 15-20 minutes. I had the coil replaced again but the problem started to re-occur a few days later.
OBD shows cylinder 2 misfire (and at some point also showed an air/fuel ratio mismatch, I assume from unburnt fuel from the misfire).
What could be the possible issue and what can I do to diagnose the problem myself?