I keep getting the p0420 code that indicates either low catalyst efficiency or a faulty downstream o2 sensor. Obviously, the CAT is possibly the problem. But I suspect the CAT efficiency could be undermined by another problem, such as burned oil or unburned gas, so I want to find and fix any such issues. The car still drives fine, so I keep driving it, but I have a SMOG test due soon.
Symptoms:
The P0420 code has lit up the CEL 8-10 times but in one situation only: steady cruise, light throttle, freeway speed, after the car is pretty thoroughly warmed up (at least 8 or 10 miles of driving). It has not yet come on when doing any of my typical hypermiling, such as P&G, which involves high load and short runs.
Diagnosis (pictures!)
[I will edit this to upload pictures in a few minutes]
Blackened
upstream O2 sensor:
This sensor was cleaned throughly to near new appearance about 2500 miles ago.
Slightly plugged CAT honeycomb:
The dark spots are probably plugged honeycomb, and across the entire surface it looked like this. I cleaned this CAT with the acid wash method in 2016 and the honeycomb was all uniform light grey and clear afterward.
Fuel fouled plug #4:
All four plugs looked about the same, like this one, but this one (#4), was wet and smelled of gas, too. The plugs were all used but clean and gaped properly about 2500 miles ago.
Possible causes.
As steady cruise, I have also noticed the longterm fuel trim gets lean by a percent or three. So I have been wondering about a rich fuel condition. Obviously, the fuel fouled spark plugs suggest that.
I have wondered about a partially plugged up CAT, which obviously kinda seems to be the case in the photo.
Next steps
I already tried cleaning the O2 sensors. That solved nothing. So...
1) Test spark plugs and wires and replace the plugs if indicated.
2) Maybe wet compression test to look for oil blow-by.
3) Maybe injector test to look for leaking
4) Clean catalytic converter and O2 sensors.
YOUR THOUGHTS?