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Benny4jesus 03-21-2019 02:44 PM

Don't lose hope. P0420 and P1273 can go away! :o)
Hey guys,
I had a P0420 code for several years.
At the time, and with my lack of knowledge, I didnít care about the P0420 code ÖI thought it was only emissions. The code IS for emissions, but it can ALSO mean other problems exist too! Iíll explain later.
So I drove it like that for maybe 35k miles, maybe 40k.
A surprise pregnancy (no. 5) sent us over the baby seat limit for our larger ride. Lol
Alas, we had to buy a new car Öand sell my faithful Altima (now at 201,000). This made the P0420 code more of a deal breaker. I wouldnít get as much with the engine light on.
And so began my journey to a CEL-free dash!
I started with Bluedriver. So glad I did!! I was able to see my fuel trims!!!! Life changing tool!!!
I was at 9% on my long term and 17% on my short term!! This meant I had a few leaks!
I was opened up to the world of not trusting CEL lights! Curse you Bluedriver!! Lol
I started with a starting-fluid-spray-test and found the first leak. Turns out when I had replaced my PCV valve, I had dropped a small ring gasket that sat in the PCV-hose-to-intake port. Fixed that and watched the STFT drop to 14%. Still not good enough. Read that a lot of Altimas have cracks in their exhaust header. Checked that and found TONS! TONS! Thereís my problem!
Fortunately, the cracks and the P0420 code are the same part. My destiny was calling me! I had to replace the catalytic manifold to WIN! Lol
So I first tried a $100 aftermarket catalytic converter/exhaust manifold on Amazon. Didnít even come close to fitting! Should have known.
Returned that and bought a $350 ďOEMĒ one from OíReillyís. That didnít fit too well either. Ugh!
Engine side heat shield didnít fit, so I left that out. Had to modify a few heat shield holes for the radiator side.
Had to use spacers and different bolts to match the manifold mounting bracket up near the bottom. Finally got it in. What a pain!
Also found out that my exhaust pipe bracket had been squashed from coming down off a curb from our yard or something. So I replaced that and it helped the pipe meet up with the new manifold even more. Still wasnít OEM though as OíReilly said it was. Pshshh.
Anyways, so I started the car up.
P0420 gone! Boom. Within the first drive cycle! LTFT went down to 0%! So awesome! STFT went to 9%. Nice!
Hereís where the crap hit the fan!
The dreaded P1273 started popping up to say hi. Ugh!
Went through a few fuel trim clearing procedures (including unplugging the battery overnight) and it just kept popping back up.
Tons of sites were saying it was a lean issue. Injectors, bad sensors, yada yada.
Tried replacing my sensors a few times. Both upstream and downstream! Nope. It came back.
Found on YouTube that ONE mechanic was saying this code also popped up from an EXHAUST LEAK after the flex hose joint. I had remembered that after I installed the new converter manifold that the flex joint just didnít feel as nice of a fit when I had put it back together. It was tight, but just didnít feel right.
So I looked at that while the car was running and sure enough, noticed that I could hear the engine when I put my ear up to the flex joint. There was a smaaaall gap where O2 could get in and throw the readings at the 2nd sensor! Not enough to notice a huge difference on my Bluedriver (2nd sensor was showing 0.265 volts), but enough to supposedly throw my P1273 code.
So I installed a few rings around the male pipe connector on the manifold to help lift the curved side of the flex pipe gasket out a little further to fill up that gap I was talking about. After a few rings that I had cut out of a tin sheet I was able to make it mesh good to work with the new manifold and old flex joint and pipe. Bolted it back up. Started the car. No more noise.
The car sounded better than it had in a long time! No cracks in the exhaust, no flex pipe leak!
Performed the fuel trim history clearing procedure.
Man oh man I canít tell you how pleased I am!!!! LTFT is 0%! STFT moves around from 0 to 6%!
Been driving the car now for a long time and no codes!!! No P0420, no P1273!
Engine purrs!

Hereís my final readings:
(range includes cold start to warm engine, at idle and 3000 RPM)
STFT 0 to 6%
O2B1 0.12 to 0.9V
O2B2 0.28 to 0.31V
So what did I learn? Hereís the main things that will change my mechanic life forever:
1. Catalytic converters should last for 200,000+ miles with a well maintained engine. If youíre cat goes bad, you probably have other problems that just havenít thrown a code yet.
2. You can have small problems with no CELs that can turn into big ones!
3. Fuel trims can really help diagnose issues!
4. The 2nd O2 sensor is not JUST used for exhaust emissions! The ECM uses BOTH O2 sensors to determine fuel trims (and lean or rich fuel conditions).

So what did this mean for me? That I will fix catalytic converter problems much sooner from now on if I plan to keep my car around past 200,000 miles.
If your engine is running rich for a long time it can damage other parts, and eventually expensive ones. Iíll admit, emissions arenít my biggest concern. But I do care about engine longevity.
All that said, if you have a P0420, P1273, or P1274, hopefully my story will help save you some hassle.
Take care,

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