P0420 code after replacing catalytic converter, slightly overfilled oil

Like many Gen 2 owners I had my cat stolen recently. I replaced it with a well-reviewed but cheap $144 aftermarket unit on Amazon (look up "Autosaver88" for the exact model) but after maybe 50 miles of driving I got a P0420 code while on the highway today.

Around the same time I had the cat replaced I also had Meineke change my oil. They slightly overfilled it (looks like about 1/4qt assuming the distance between high/low on the dipstick is 1qt). I assumed that wouldn't be enough to harm anything, but is it possible it's damaged my catalytic converter?

Or is it more likely it was just a bad quality unit to begin with? The Amazon seller claims it's EPA-certified but who knows. Still, it did seem to do fine for the week or two I've been using it across several trips.

Any advice? Am I reading the dipstick correctly and is just ~1 cm above the fill line nothing to worry about?

 

I bought a $24 O2 sensor off Amazon as well (not OEM but also well-rated). I can't post the link yet (just hit 4 posts), but it's Amazon's choice for "Automotive Replacement Oxygen Sensors by SFY".

FWIW I checked the voltage via the OBD2 and it looked to be working fine (IIRC, 0.8V steady) after I installed it. Maybe I could have torqued it better (just snug, not to spec). Didn't get a P0137 code so I figured it's a good one.

By the way, I just saw on page 286 of the manual that the min/max interval is about 1.5qt:

So a quarter of that distance above would have been about 1.5*0.25=0.375qt extra. Still probably not enough to cause damage, would it? I saw elsewhere on Priuschat that Gen1s are very sensitive to oil levels. What about Gen2?

 

I just re-checked the oil and it's actually about a third of the way up the dipstick, so about 1.5L (1.6qt)/3=0.53qt extra. Out of a 3.9qt capacity that sounds like it could be bad

I should mention too that the O2 voltages looked fine when I tested ~50 miles ago. It's possible driving the ~50 miles since then with the extra oil that something has changed... Hopefully the sensor hasn't gotten fouled up and ruined already.

I guess I'll have Meineke drain the oil first thing (and see how much responsibility they'll take), then maybe try some Cataclean on it, clear the P0420 code, then drive some more and see if the code comes back? Hopefully the cat can still be saved.

Here's the O2 sensor I bought now that I can post links:

 

From this thread it sounds like things could be (or get) a lot worse:
Oil overfill caused complete shutdown | PriusChat

My oil level is a half-inch above the max-fill line, which is apparently enough to get into the intake manifold:
Oil overfill caused complete shutdown | Page 2 | PriusChat

Maybe I'm overthinking this but really wish they hadn't overfilled...

 

So do you think it's unrelated to the oil overfill? I was considering opening up the intake and checking for any excess oil to be sure, like shown here:

Would be great to know if it's just the cat/O2 sensor vs potential engine damage.

The oil level hasn't actually changed and it's only been driven 50 miles. Presumably if it were burning oil in the catalytic converter and clogging it up, I'd see a drop in the dipstick level?

 

Somehow the level at the shop was reading at 1/4-1/8inch above max instead of 1/2inch. They removed about 1/8qt and now it looks like it's just below max.

Don't know what happened to the oil I saw before. I was pretty sure I read it correctly - read twice on flat ground, wiped stick clean before, engine cold in the morning or 1hr after driving.

Hopefully the excess oil was just a non-issue. Cleared the P0420 and going to drive it another 50 miles and see if the code comes back. Not sure if it is the cat or the non-OEM O2 sensor, but if it does keep coming back I may just try installing a spacer: Catalytic converter code(P0420)?Solution inside | PriusChat

 

Well the P0420 code came back after <10 miles of driving, maybe a minute or two after entering the highway a second time.

I checked the code right after I parked and saw the (downstream) O2 voltage was holding at 0.78V steady. I took it around the block at low speed and it saw it did jump around a bit to 0.72v, 0.6v, later as low as 0.41v, 0.3v. Shouldn't it be steady? I don't think I had turned the car off but it was in park briefly.

I then also checked the cat temperature sensors and got readings around 780F and 720F for sensor 1 and 2 respectively. I read that a 30-100F difference is a good sign the cat is working. It probably cooled off quite a bit in the ~5 minutes since the code originally set on the highway. Would getting the temperature readings at that point tell me anything?

I would normally try getting my OBD2 to record the data while plugged in on the highway to see what exactly is triggering the P0420, but I've been getting the mysterious set of "Communication lost" codes when my reader is plugged in for a bit: U0293 u0100 u0111 u0293 p0a1d | PriusChat (Maybe that's just due to my 12v being weak)

 

Any recommendations for readers that would be known not to trigger these codes? I do have a mini-VCI cable I've been meaning to try once I can get Techstream installed, but I've read they can trigger similar codes.

The reader I'm using is a cheap ELM327 clone from eBay, but I'm skeptical that's the problem, since I never actually had those U0100 etc codes using the OBD2 several times before over 3 years. After having codes read at the shop, the mechanic's reader triggered some similar codes too (unless they also used a cheap one?). I wonder if it could actually be a weak ground connection somewhere or weak 12v? or maybe a flaky connection somewhere from rats chewing wires?

I have had some weird electrical issues where having something plugged into my passenger-side 12v caused my combination meter to go out once, but it's been fine after removing that device (think it was a 12v splitter).

The other weird thing is the codes once appeared *after* the OBD2 is was taken out and driven a bit. I had a scare where the red triangle appeared while I was sitting at a red light. Luckily turning the car off and on brought it back into READY mode... If things get that bad again I might have to get rid of the car. Hopefully that was just a one-off.

 

So if it is just the sensor, could I just put a $5 spacer on? (like described in this thread: Catalytic converter code(P0420)?Solution inside | PriusChat)

Obviously not ideal but it seems like everything else would be working fine right now. I just want to get rid of the code (and not have the light on all the time so I know when something else goes wrong down the line). There isn't a way to tell the ECU to (permanently) ignore a specific code, is there?

 

Following up - it's been about 2 years with this aftermarket cat (and P0420 code). I had some misfiring when my spark plugs went bad around 100k miles, and I replaced them about 5k miles ago. I suspect the cat may be clogged now though. P0420 code comes back almost immediately after clearing instead of after 20-50 miles.

power and mpg is a *bit* lower than I remember (~40 mpg, despite a fresh OEM hybrid battery). I also got a CO meter in the cabin and measured that occasionally CO levels bump to 50+ ppm and sometimes even higher (only for a moment though, I keep the windows down); maybe due to backpressure building up from a partially clogged cat, sometimes enough to leak into the cabin? It's not consistent though; most of the time CO readings are 0ppm.

Thoughts on how to proceed? I'm not totally sure it's the cat (might double-check the torque on the spark plugs); I had the gasket and welds (where the OEM was cut out) checked and they couldn't find a leak there. Maybe I should have the exhaust manifold checked or have a smoke test done, or get a vacuum gauge test done? Trying to confirm if it's the cat before I replace it. I hope it was just clogged by my old misfiring spark plugs and not caused by something else. I don't think the engine has been burning oil between changes; the level is steady.

I was also thinking to try running a bottle of Cataclean/Oxicat/fuel cleaner through it to see if that helps (but heard mixed things about them damaging things like the fuel bladder), is it worth trying?

 

Definitely no oil consumption; just checked and as usual the oil level hasn't moved at all since my last change.

How quickly would misfiring foul a cat? Looking back to last year, I only drove <20mi after getting the P0301/P0302 code set and replacing my plugs: Occasional Jerking/Shuddering, Lumpy/Rough Idling Engine | PriusChat The cat seemed okay then; could it have just gotten slowly clogged with a P0420?

 

Had a muffler shop do a backpressure test and they measured 0.6psi (they drilled a hole instead of plugging into the o2 sensor). That didn't seem very high to me (referencing https://www.carparts.com/blog/how-to-tell-if-the-catalytic-converter-is-clogged/#Testing_for_Backpressure), but it was enough for the shop to recommend replacing the cat.

Thoughts? That much backpressure doesn't sound like it would cause the exhaust leaking I'm seeing.