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Discussion Starter #1
'05 Tuscon V6 coded a misfire in cylinder 5. It previously coded an issue with the o2 sensor in bank 1, sensor 2. I didn't have access to a live scan tool and that sensor is a pita to check with a volt meter so I checked the spark plugs first. The plugs in 1, 3, and 5 had some carbon deposits like it had been running rich, but they were also extremely worn like they had been in since day 1 and for 130k miles. The plugs in 2, 4, and 6 seemed to be running a bit lean (white deposits) but not too serious. Bank 2 o2 sensors have not reported any issues. I replaced all the plugs and checked the wires' resistance. They were perfectly in spec with rubbery, supple insulation so I left them. The worn plugs were enough to cause the misfire, but it still doesn't quite explain that the downstream o2 sensor had previously had issues. I borrowed a live scan tool to check the sensors and the downstream sensors in both banks 1 and 2 aren't quite as steady as I think they should be. They seem to mirror the high/low voltage fluctuations of sensor 1 in their respective banks but the fluctuations are only about half as frequent. Is this normal or is it pointing to failing cats?

444299


444300
 

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Discussion Starter #3
Your misfire(s) ruined the cat.
Since it has two cats, one for odd cylinders and another for evens, why are both downstream o2 sensors reading funky? There was never a misfire in 2, 4 or 6 and bank 2 sensors have never been implicated.
 

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Perhaps the first O2 sensor for the bank with cylinders 2,4 and 6 has been faulty and driving the mixture richer than it should be. The is hard on those plugs and the associated cat. Exhaust system leaks [even small ones] anywhere from the exhaust ports to 12inches past the 2nd O2 sensor can mess up fuel management and O2 sensor readings. Might be a good idea to just change the upstream O2 sensor for cylinders 2,4 and 6. Hopefully this will get you more time out the existing cats. The engine was totally warmed up when you took these readings?
 
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