Please, look at this at your own risk - these are for my reference and I shall not be responsible for anything you do or do not do with the information here. Working under the car is dangerous and the reapir may turn expensive or dangerous if you screw-up.

My '02 Prius, which I bought used when it was at 116,000 miles has been giving me repeatedly code P0420 - Catalytic Converter Efficiency Below Threshold. It now has 120K and it seems the problem will not go away.

Testing the front and back oxygen sensors seems to indicate they are OK - front swings b/w 0.1 and 0.9V approximately and the back changes less vigorously at say around 0.6V plus/minus 0.1V (e.g. from about 0.5 to 0.7V) but still swings more than it probably should, especially if the car is under load (e.g. uphill or driving faster). This seems to point to an actual weakening of the cat converter, rather than a bad O2 sensor.

The Toyota cost for the cat converter assembly alone is over $1,600 USD in the US. Federal regulations seem to prohibit the sales of untested used converters thus limiting the availability of potentially cheaper and perfectly good replacements from local junk yards. Aftermarket alternatives are very limited as well - I have only seen one example of a successful replacement of the first of the two in-line converters here http://www.artsautomotive.com/PriusConverter.htm and I have not researched what specific converter they used nor the cost.

I was lucky enough to find a used exhaust off a '02 at supposedly under 30K miles from XVipers.com - they were helpful and shipped fast. They have lots of used Prius stuff, so check them out at their web site or on eBay. For $350 + $100 freight shipping to where I am I got the cat converter and the muffler off a low mileage donor car. Shipping is high since there is over six feet of monolite piping for the first and second cat and the resonator, plus a separate muffler that itself is about 18 lb and close to 60" long).

I hope to recover some of the cost by selling the old cat converters to a recycler. Seems that they can fetch $30 to $80. I hope to find a buyer - if thieves can sell them, so should I, shouldn't I? -;)

The replacement was straightforward and it took me about 3 hours including the setup and clean-up in my garage and no use of power tool. All that's was needed was a good 22mm open wrench for the O2 sensor (or a specialty O2 sensor wrench if you have one instead), a 14mm socket wtih about 10-15" extension for the two bolts that hold the cat converter to the exhaust manifold, a breaker bar (I used 24") and a botle of liquid soap.

Did it work? The car ran fine, but it took some restart cycles to establish readiness of the exhaust system to determine if the code is gone. For me, this usually takes a couple of hundred miles, so I did not immediately know if it worked for sure.

Most photos are self-explanatory and many include some text.
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Update #1: Test drive after the install. The exhaust system readiness "complete" status was still "N" meaning I had to go thru several more start-shut cycles on the car before the computer decides if the emissions system works or not. Initial voltage readings I was getting off the second O2 indicated the new cat was doing a better job then the old one. With the old cat I was getting swings of 0.2V or more under the test conditions specified for troubleshooting P0420 trouble code. With the new one it seems to stay within 0.05V or less. This is about 4 times less change than it used to be. Measurements were taken via a "live data" capable generic OBD-II scanner while driving at 55 mph on the highway for more than 5 minutes and maintaining constant engine load.

The center voltage value varies according to engine load and RPM but the swing value around it seems to stay within a fairly low margin at all times.

So, the voltage swing has decreased at least 4 times if not more and
that can only be a good thing. Hope the car computer agrees with me and
does not complain again about P0420...
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Update #2: Two days later...
The Emissions Control System has apparently completed its checks and shows a "ready" in the OBD-II scanner's I/M screen. This means the car computer likes the new cat converter and it is apparently working. If no trouble codes show-up after a couple of fill-ups (800 miles or so) I will consider this repair successful.
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Update #3: 2,000 or so miles later: still no P0420 DTC, which is good. The replacement cat seems to be working fine. Drove the car in rain, snow, cold (20F) and warm (80F) and it seems to work just fine.

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Update #4: Now, at about 6K miles later, the car is still fine and just past the 127K mile mark on the odometer. Knock on wood, may it go another 100K miles trouble-free -;)

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Update #5: Now, 141K miles. Knock on wood, my repair still holds after about 25K miles. I do get the P0420 flash from time to time if I go fast on the highway with overloaded car and two kayaks on the roof. But does not come if I just drive the car normally with no roof rack on it. I think the computer is just too sensitive or the sensors may be a little off-spec or the cat converter may indeed be getting weak again...

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Update # 6: 9/23/2010. Now, at 170K miles, the car is getting the same P0420 code again, pretty much every time I get on the highway and drive fast. I now always have my rack and at least once a week a kayak or two on top so I do not know if it will clear if I take the rack off like it did before. I suspect it may if I baby the car and never drive aggressively, but the hope is slim - the code seems to appear much more readily now than it used to 30K miles ago. I suppose the "new" converter got weak[er]. Or may be my sensor is a little off so the computer thinks there is a problem. I am basically ignoring it and if I get tired of looking at the check engine light, just resetting the computer.

Would be interesting to see if others have similar experience. My "replacement" exhaust system was not in great shape - had I gotten a new one instead of a used one, I guess I would have more miles before that problem appeared again ...

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Final Update #7: 4/4/2011 - with close to $180K miles, replaced the front (bank 1) oxygen sensor. The replacement 02 sensor is a $35 (shipping included) 4 wire unit off eBay. Seems to have fixed the problem once again. The voltage readings off the rear (bank 2) sensor looked normal and I hope the new owner does not get a nasty surprise a few days down the road. I have just sold the car so likely won't be able to follow-up and find out how long that fix lasts. I imagine that replacing the front (upstream) O2 sensor like I just did would have been the smarter thing to do first as (I'm told) these things get "lazy" with age and may trick the car computer into thinking that the exhaust is bad before it actually is bad enough.