Whelp, I got a code reader, and I've finally narrowed it down to two things it could possibly be. The cat, or an O2 sensor. Part of me is leaning towards the cat being bad, mainly because all the O2 sensors are fluctuating. That being said, it could be a rogue O2 sensor that's messing with my readings. What do you think the problem is?
I would try a bottle of Seafoam and some some fuel system cleaner first. Cheaper than both other options.
I've never had a catalyst-related code persist after replacing all the sensors at once (upstream & downstream). YMMV.
Downstream O2 codes can also be caused by bad connections, often the connector is under the car.
How many miles are on it? Sensors only last 100-150k miles before they start going out of tolerance and reporting lean, causing the car to run a little rich. Not sure if that will trip that code though.
180,000 miles. Since the code pertains to bank 1, I'm assuming the catalyst has failed. Bank 1 reads in the 500s while Bank 2 reads in the 400s (whatever that means). Could the O2 sensors fallen out of tolerance?
At that mileage, shotgunning it with OEM (probably NTK, check rockauto) O2 sensors wouldn't be a bad start. If you have live data capability, look at the long term trims (LTT) values when running, more than +4-5%(i.e. trimmed 5% richer than baseline) and the upstreams are definitely shot. It will get better gas mileage with new ones if the upstreams have gone lean.
Ignition system tuneup isn't a bad start either.
What car are we dealing with here? Best to note that, talking in engines codes is poor form unless it's some sort of hot rod.
In reply to Kenny_McCormic:
2006 Toyota Sienna FWD. I'll check the LTT values.
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