I have code p0420, tried cataclean - it didn't help. A month later the code disappeared. The car is Fit with 97k on it. Never had loss of power or misfire.
I cleared the code and ran a test, no faulty codes. I ran also live data test.

Ignition on > clear code > Start engine > keep 3000rpm at idle until radiator’s fan comes on>test drive > keep constant speed in range 45-75 mph for 5 min (75-120 km/h) If screen shows passed – it is OK
Observe live data for first sensor (closer to engine) – voltage should fluctuate several times per second around 450mV, from 200 to 700 – means sensor is OK

I used ML329 scanner. I was not able to find a list with all acronyms used in the device and the values or range which are normal.
I would like also to compare data measured for some parameters, for parts which could be a reason for this cat code -
Possible causes
1. Faulty Three-way Catalyst Converter Bank 2. Exhaust Tube Intake Air Leaks 3. Faulty Oxygen (O2) Sensor 4. Faulty Fuel injector(s) 5. Leaking Fuel Injector (s) 6. Faulty Spark plugs 7. Improper Ignition Timing 8. Faulty Engine Control Module (ECM) 9. Dirty Air Filter

 

Wait for code to return. Erase code again. See how long it takes for the same code to return again.

 

It disappeared for 8 days now. How this can help to troubleshoot?

 

It's been 8 days and the code has not returned? Keep driving!

Like I said, See how long it takes for the same code to return again

 

Ones it disappeared and came back in 2 days. 2 weeks later disappeared again for a month and now check engine light is on again.
I will edit this to enter changes chronologically
on 30 days
off 2 days
on 15 days
off 30 days
on 7 days
off 30 days (96870mi 04/25)
0n currently (97450mi 05/25)

 

P0420 still returns? Probably gonna need a cat

Now I'd want to know what caused the cat to fail ........ Need to fix any running problems so the new cat won't get ruined right away

 

This is why I haven't replace it yet. It looks no one from the colleagues here has experience with this issue.
Found spark plug #4 loose. Maybe it caused the coil #4 to change its color. The gab was good to all of them. Tightened the plug and check enbgine light went off. Hopefully not temporarily.
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from R to L (1,2,3,4)