The car is a 2002 Honda CRV with the 2.4 VTEC engine. Had it about two years, and I've done a fair amount of maintenance on it at this point.
It's got a nagging, intermittent p0420 code. And possibly related, the gas mileage is about %20 below where it should be, getting appx 22 mpg out of expected 28 mpg on the highway.
Last spring I removed the cat and ran a can of oven cleaner through it, rinsed it out with a LOT of rubbing/isopropyl alcohol, plugging it and really shaking it, followed by several rounds of jetting it with the water hose, getting it from all directions--fore flushing, back flushing, through the o2 bungs, etc. It certainly wasn't clogged, the honeycomb looked good, seemed to have good airflow, but it didn't resolve the p0420 issue.
Since then I've changed the spark plugs with the preferred NGKs, factory gaps (double-checked) as well as the air filter. I used about a half-can of CRC Industries Intake valve cleaner directly in the throttle body, spray cleaned the MAF sensor using the appropriate solvent, and ran a few courses of Techron complete fuel system cleaner at fill ups.
The old spark plugs didn't look too bad ..but it might be worth pulling one now to see what about 5k miles of driving has exposed it to.
Finally, this week I replaced both O2 sensors with new NTK (OEM) sensors. Yet the P0420 remains intermittent.
The Important Part
When I used the CRC Intake cleaner a couple weekends ago, spraying it into the throttle body with the engine at about 1800/2000 rpm, on a few occasions I noticed a kind of "burp" of smokey air emit from a passage that exited the engine and would have gone directly into the air box ..which was removed to use the solvent. At first I thought it was just an indicator that it's burning a little oil, which I've suspected (confirmation bias) because toward the end of the oil cycle it's always a smidge low.
However, contemplating this, I've started to wonder if the smoky "burp" could be PCV related ..and sure enough this engine has a PCV valve. And indeed, there is even a review for the genuine Honda PCV where a user noted replacing it "..stopped oil from being drawn into the air intake box." Another buyer of non-OEM PCV even noted their gas consumption rebounded.
So the question: Is it reasonable that a PCV valve in a failing or failed state could be the reason for an intermittent p0420 catalyst efficiency trouble code?
Essentially, I'm trying to rule out any potential causes of the p0420 before throwing money at the cat. And my gut feeling is the reduced mileage is a good indicator that maybe the cat itself is not the culprit.