Ok, this is interesting. My old Prius has been throwing P0420 errors from time to time. Makes sense, as the car has well over 200k miles on it. Oddly enough my 2002 Sienna has also been throwing these, and thus passing emissions is a pain. A few months ago the Sienna started running very badly, and threw major P0303 misfire errors. Sure enough one of the coil-on-plug devices had failed. So I swapped it out, Sienna works fine. Onto the Prius. It's been throwing 0420 errors and I took it to the dealer (big, bad mistake). They said new cat, $1400. Then I pulled the codes and noticed an interesting one. P0302. Random misfire on cylinder 2. So I thought about it: If the COP units were going bad they would fire intermittently or may not produce as strong a spark. That could lead to un-burned fuel in the exhaust maybe.... So I spent $60 bucks and bought 4 new Denso plug coils on Ebay. Replace them (on the Prius it's a 10 minute job), reset the computer, and took it for a single, 15 minute drive. Then a stop, and another little drive. Came back home and checked the codes. Not only no codes, but it had gone to GREEN. Immediately. Normally the C test would run for weeks, but every test had passed and every code cleared. Wife took it to emissions, passed no problem. I'll see if the error re-appears, however there does seem to be a correlation here between those coils and a P0420. The big test might be to swap all of them on the Sienna and see if it passes diagnostics as well..... It's a data point. And a $60 repair is much better than a $1,400 repair.
Granted it's only been a day, but I have been swapping a few things around. I found that swapping in the old #4 coil unit would give me a P0420 right off the bat. Swapping out problem goes away.
Ok, it's been a week and still no problems. Car went green again and is fine with that one coil replaced. Now the big questions are: 1) Why 2) Will this same fix work in my 2002 Sienna which is throwing P0421 (bank 2)? 3) How many catalytic converters have been trashed over this 4) How many people have gone bats because they replaced the cat and 0420 came back after a few years.....
Couple of weeks and over a thousand miles. Still no problem, car seems happy. Better than pulling the whole cat assembly.....
The dealership's "solution" was completely off-the-mark. Maybe email the service manager, even just link this thread.
This thread is an excellent reminder than when a computer in the car logs a trouble code, you cannot jump straight from what the code's "fortune cookie" says to thinking you know what the problem is. The computer setting the code only tells you what the computer is seeing. It's still up to you to think through all the reasons the computer could be seeing that and eliminate the possibilities one by one. And the one that remains can always turn out to be a $60 one and not a $1400 one.
