[jiantao]

It might not have anything to do with gas, and it's something completely different. Maybe the cat or O2 sensor, as you mentioned.

Here's my experience with P0420 on my Camry:
2017, around 135K miles - CEL comes on, P0420. Cleared the code, stayed off for awhile. Took to inspection at 137K, passed, good sticker for 2 years.
2018 around 138K, CEL comes back, erased code, comes back months some later around 140K.
2019 around 147K, due for 2 year inspection again so erased code, drove for a little bit, passed inspection. Shortly after, CEL back again. Now the code says oxygen sensor (it was P0420 - catalytic converter for years).
2021 around 155K, due for inspection again but this time clearing the codes doesn't work. CEL comes back before the readiness monitors reset. Replaced the offending oxygen sensor which is causing the CEL (I think it was P0136). FIXED - CEL does not come back.
Fast forward to 2024, around 175K, the CEL is back again and is once again P0420 (7 years after it first came on). Developed an exhaust leak about a week after the light came on this time, not sure if it's related or not. I've tried clearing it a few times since and it always comes back. At 178K now.

Going to try cataclean next.

 

[jiantao]

Update:
Installed new fuel pressure regulator.
fuel pressure test results: same as before. pumping at about 60psi, resting at 46 psi. dropping pressure to 43.75 psi in 3 minutes. engine off. the FPR is in the fuel pump assembly in the tank.
I checked the cylinders - all dry.
I checked the FI's after priming fuel system a few times - dry.
With engine on, the pressure stays constant with and without depressing gas pedal at about 58 -ish psi.
I didn't check for error code (P0420), nor deleted it, yet.
ANOTHER SCARY OBSERVATION:
loss of coolant - I lost about less than half a gallon in 1.5 years.
I got me thinking. what if the coolant presence in the cylinders affects air/fuel ratio and computer adds more fuel, thus increasing STFT and LTFT, thus, throwing P0420 code?
Possible faults - head gasket job was not done right 1.5 years ago.
I still have to check two things:
1 - presence of coolant in the cylinders after driving
2 - presence of coolant in the drained oil (honestly I doubt that there is any). I had already 2 drains and didn't notice any water/coolant.
What are your thought, guys?

EDIT: next step in to pressure test the collant system.

I'm sure you discussed it in another thread, but why did you need to replace the head gasket? I don't believe they're known to be common issues with the K24.

I recently sold one of my 2001 Camrys and replaced it with a 2010 CR-V like yours, although mine has a fraction of the miles yours does.

 

[jiantao]

Update:
A new "Rod Seal" between the master cyl and the brake booster is installed. There was some rust in its cavity, which was "treated".
Result: the brake booster doesn't hold the vac.
Verdict: need new brake booster.
Thoughts: I don't know why the brakes work and the pedal is not hard.
Oh, well....
The STFT are still high in their 30's.

Interesting, I didn't expect that to be the issue, but it makes sense.

I had a bad brake booster on a 2011 Mazda CX-9. They were common with 2010-2012 CX-9s, I think. It got real bad and started hissing when hitting the brakes. That car even stalled out on me at a red light due to the brake booster.