I did a search and found a number of threads relating to this, but no finite solution that I saw.

My car has 160k miles and recently the check engine light came on and it was the P0303 code. I replaced the coil packs and the plugs with oem, and now a few days later the same code is back and the car is running poorly, just like it was prior to the new plugs and coils. What else could I do?

I'd also add that the check engine light comes and goes, and it runs poorly after start up for a while, and then after it's been driven for a while, like an hour, it starts running well again, and the check engine light will go away, only to come back after it's been sitting overnight and started again.

Thanks to anyone who takes the time to help.

 

Valve lash adjustment? If it's been more than 50K miles check it.

 

Could be a wiring problem somewhere, some critters eating away at something? Anyway, dealer replaced my injectors under warranty ($800 for parts alone) on my 2015 and check engine light went away. Take it to a reputable mechanic to diagnose the issue.

 

+ 1 on the valve lash.

A tired battery/dirty connections in the + battery box fuse holder can cause this as well.