Ok so I was just driving one day and my check engine light came on...I had a mechanic take alook at it he said everything seemed to be completely fine...it comes on about 5 minutes after ive been driving..and if i put it in neutral and turn it off and back on it goes off and it doesnt come back on until I put the car in park and turn it off again...its been doing this for a whiel..it runs fine just like it used to..I do need a valve adjustment...that wouldnt be it though right?

 

check your ecu code first. that should answer your question

 

most likely O2 sensor, but check the ecu for codes.