Kia Troubleshooting Questions? Ask a Mechanic
Hello Im Chris and I will assist you. As I read your story you said it ran fine until you changed oil and spark plugs. Im wondering about the spark plugs. Did you try replacing them again?
Also is your tachometer acting up?
Also keep in mind that a p0604 is usually a bad PCM. Clear the code and see if it returns.
If it returns it may need to be reprogrammed or replaced.
Thank you and please rate me so I can get credit for my time.