I bought this car back in Oct. Code for EGR popped up. I removed the upper manifold and cleaned the port and replaced the EGR valve. Then I got a P0420 code for Cat Below Efficiency. I replaced the clogged Cat and the Rear O2 sensor. Started the car up and the readings were fine. The next day it throws the same code. I wondering if the front O2 sensor needs replacing also.

I also recently replaced the valve cover gaskets, including the grommets and spark plug ones. I noticed the plug wires were not NGK ones and I'm assuming the plugs are not OEM/NGK ones either. I have been told that this will cause the car to run hot even though the thermostat is working properly and the temp gauge is fine (below half). I notice that the cabin gets warm even with the heat off.

Will this cause the car to run too lean and throw a P0420 code? This has been never ending and I would like to get this fixed so i can get my car inspected...

 

Try fuel filter and having the injectors cleaned next.

 

Its best to replace both oxygen sensors when replacing one on cars with two oxygen sensors.


You might want to check for an oxygen leak using seafoam before replacing the front oxygen sensor just to be on the safe side.