need help with check engine light
 

I have a 2004 8 with a check engine light. I changed all the coils and spark plugs and the check engine light is still on. I took it to auto zone to get the codes but I only got p0138 and p0420. Since the cat is bad can that make my car sound like its misfiring? Also to fix these codes I know I need to buy an o2 sensor but should I get another cat or is it better to buy a midpipe? One last question can bad fuel injectors be a problem also?

You ask>>should I get another cat or is it better to buy a midpipe?
I say>> You did not provide enough info.

Better than what?
If you get rid of the cat and get a midpipe, You will always have a CEL lite.

Can you pass inspection with the CEL on?

I would replace the sensors and the cat. Mileage and maintenance history?

I got the car three years ago with 74000 miles and it broke down like 2 months after I bought it. I took it to Mazda and they wouldn't cover my car for some reason. So I payed another shop to change the motor. They have changed the motor on my car twice because so I never changed spark plugs or coils assuming they did when they changed the motor. A week ago I changed my spark plugs, coils and my oil to royal purple 5w30. My car is running strong but if I listen to it idle it sounds like its misfiring but could it just be my bad cat that's making that sound.I do redline my car at least once every time I use it. I don't have much maintenance history because its been in my garage or at the mechanic most of the time. I'm changing the starter this weekend and hopefully the 02 sensor. I'm just hoping its not the motor this time but just the cat and the sensor. I don't have any misfire codes.

I would not run the car if the cat is bad as I think you would kill the engine pretty quickly.

Turns out it was just a bad gasket in my cat.