Diagnosing p0420 and loss of power

I've gleaned so much info from these forums over the years I've never had to make an account but here we go. Thanks everyone for your previous questions and answers.

I'm driving an '07 Prius with 210k miles that has lost power suddenly on acceleration. It starts and idles fine but when accelerating without the hybrid battery helping it has very little power. Much less than it has in the past.

The Facts: The Prius has burnt oil for about 100k miles, though diligently topped off. I replaced hybrid battery last year and replaced spark plugs two years ago (along with a lot of other 60,000 mile maintenance items). It's had a P0420 code for a few months, no other error codes thrown after power has been lost. So I started testing for a plugged cat. A vacuum test at 6,500 ft/sea level idles around 15 hg/cm, increases and fluctuates slightly between 18 and 15 when held around 2500 rpms. I removed the downstream o2 sensor (can't easily get to upstream) and the vacuum test was the same but without the fluctuations, it held steady around 18 when around 2500rpm. When the car is at operating temp and held at 2500 rpms the voltage wide range bank 1 sensor 1 is pretty steady at 3.3, while the voltage on downstream o2 sensor fluctuates rapidly between .1 and .8v. Catalyst bank 1 sensor 1 is around 630F and bank 1 sensor 2 is 360F. At same conditions the temp in exhaust measured with infrared thermometer just before first cat is around 500F, behind first cat 600F, before second cat 500F, behind second cat 350F. Long term fuel trims in maintenance mode at 2500rpm level around 0 and Short temp fluctuates but not more than 10% in either direction (mostly being positive).

My Diagnosis: I'm wondering if you can help me make sense of the information. So I'm first trying to diagnose a clogged catalytic converter because of the p0420 code, thinking a clogged exhaust would lead to loss of power. I'd love to be sure before I buy one and slap it in, even though I know the aftermarket cats aren't expensive and pretty easy to replace. Mostly I want to solve the power issue. It seems like the vacuum test is inconclusive because the pressure didn't steadily drop as it would with a clogged exhaust, but it also didn't seem working great and was slightly improved by removing downstream o2 sensor. The voltage test seems that the catalytic converter isn't working well given the wide fluctuations in voltage while the a/f seems to be working well. I'm not sure enough if the OBD read catalyst temps should be higher in the second sensor than first, but that is what I would assume given how they work. The physical temperature test seems like the first cat is working mostly well by heating up the exhaust and the rear cat may not be functioning well, but I'm not sure how a fouled second cat would throw a p0420 code.

Thanks for your set of eyes and brains on these, and please feel free to suggest I look elsewhere to diagnose this power issue.

 

Well we can go pretty fast on flat ground because typically we have power from the traction engine. When we're going up a hill and hybrid battery is done we're maxing at around 35-40mph with the pedal to the floor. Before I replaced the hybrid battery we'd go up to ski area without hybrid and the engine would work hard but wouldn't have such low power like this. I'll try to remove the cat or at least detach exhaust from header and see if this helps. I suppose I'll have to fix the cat either way if we want to keep it cruising for a while. My fear is that it's out of timing and a much more laborious job to fix.

 

Yes this is what I'm realizing. Are you recommending replacing the downstream o2 sensor or the upstream a/f sensor with this fix? I wanted to make sure it was cat before going through the work but I'll have to replace cat anyways to fix p0420 so might as well start there and move onto further diagnostics (glup... timing?) if necessary.

 

Yea thanks, I realized that was obvious after thinking for 5 minutes but I couldn't figure out how to edit my post. Thanks, parts in the mail now. I may be back, but hopefully this fixes it.

 

So an update:
I replaced the cat and the o2 sensor and this fixed the power issue. The old cat was very plugged with carbon.

The problem is the cat I bought was not nearly as tight of a fit as most that I've seen and I can hear and smell exhaust leak under the car at the transition of new cat to resonator pipe.

I bought the funsport cat (can't post links yet) on ebay and would not recommend it at all. I'll probably contact their support, but it's probably not worth the headache. The issue is their new pipe is quite a bit larger than the existing pipe, way more than could be clamped easily, so they provide a transition piece that's even wider (slides over their pipe) and tapers to fit over the existing pipe. Neither side of this transition piece is very tight, and they only include one clamp, so I clamped down as best as I could but will need at least one more to try to get the two sides of the pipe tight.

For those of you with experience: should I use a high-temp gasket or something on the pipes here to ensure a tight and leak-free fit, or should clamps work? Worst case I'll have a muffler shop weld it on.

 

Since I already replaced the cat I don't think I'll buy another one just yet, but first try to plug up the exhaust leak with muffler sealant and another clamp. I reused the manifold gasket from the old one since it seemed like it was in good condition and better than the new one delivered.

Thanks for the likely causes list, guy_mann, the old cat had the moist, shiny oil you describe. I wrote in my first post on this thread some of the conditions and diagnosis of the prius we own and to me it seems most likely that the piston rings are worn. I understand that's the likely underlying cause to this and the cat failed because of excessive oil burning, but also recognize that I don't have the time or energy now to change piston rings. The car has 210k miles and we're trying to get what we can out of it, but I'm not sure an engine rebuild at this point makes sense for us. Though I know it'll keep burning oil and this cat will eventually get plugged in the same way.