Replaced all 4 tires last week and a couple days later, notice CEL came on even though the car drives normal. What I notice though is when the car has been parked for a few hours and once i start it up, the rpm would be around 1200 and after a few sec it would drop down real quick to about 1000. when it does that, the lights inside the car would go dim for a quick sec and the car would go back up to 1200rpm. It does this a few times. Once I start driving though, rpm idles normal.

Before what usually happens when I first start up the car is the radiator would make some weird noise coming from the fan of some sort. My guess would be the CEL is related to the radiator somehow? Not sure though. I also have to replace the valve cover asap as well due to leak. Thoughts?