P0420 and engine life

Hello, I’ve been lurking for over a year but am new to posting. I recently posted a diagnosis question and got some good feedback and figured I’d try my hand with what’s worrying me lately: my engine (and cat). I had my cat stolen about 2.5 years ago and replaced it with some garbage off ebay. Of course, I was blessed with a P0420 code for my troubles and decided to send it as I had read that it wasn’t too detrimental to drive with it.

Fast forward to about 8 months ago, I bought a new engine and swapped it in because my old one developed a knock that kept getting louder and louder. I was worried it would just die on me while driving so I learned the ropes and replaced the engine myself before that could happen. The new engine ran absolutely great and immediately boosted my mpgs from 42ish to 47+. I got new plugs, o2 sensors (OEM), and coils (denso) to ensure I gave it every opportunity to be healthy. Now, it’s developing the same kind of sound (albeit in the beginning stages). I replaced my cat and it’s giving me the code once again so I’ll just be using a spacer until i can afford to get an OEM cat again with a shield.

My big question is whether or not it’s already too late for this new engine? I’m terrified at the thought of having to wrestle an engine out once again but i dont have a clue why it’s knocking and refuse to risk driving a ticking time bomb that can leave me stranded. Does anyone here have any knowledge on this issue that can shine some light on it for me? Thanks in advance!

 

hello again! It could be one of those? I got under it while running to hear what part of the engine was knocking (without a stethoscope unfortunately) and it sounded like it was toward the exhaust manifold but obviously in the cylinders somewhere. I didnt know if my previous cat was clogged and causing extreme back pressure. Im very handy with a wrench but not so handy in diagnosing intra-engine problems. However, just so it’s known this is a true engine knock that goes with rpms. It sounds eerily similar to piston slap but is present at all engine temperatures.

 

I generally agree however the knock towards the end of the old engines life was definitely no light knock. Drove fine, still got good mileage albeit on the lower side (40-41). I would have everyone turning their heads to look where the sound was coming from when i drove it, though. It was RPM specific and definitely within the combustion chamber. Piston slap was the closest sound i could find.