Ok, this is my story. I have a stock 97 honda civic ex with 169,000 miles and two months ago it starting loosing horse power really bad that it would take forever to accelerate. It would do it all the time and it also made a rumbling noise. I had the catalytic converter replaced and it improved a lot. However, it still does it but not as bad and not as often(also makes the rumbling noise). I already changed the air filter, spark plugs, cap, rotor, and wires.Today the engine light came on showing p020 (catalyst system efficiency below threshhold bank 1). Has anyone had this problem and have an idea of what to replace to fix it? I do not want to keep guessing. Thanks.

 

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P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)