So this is the typical "inefficient catalytic convertor" code that gets thrown. It is the last one I have to deal with on my 231k 3.0L Auto Accord.

Is it the downstream 02 sensor (rear) that usually causes this? Is it the actual CAT?

What fixed your issues?

Thanks.

 

Upstream O2 sensor, exhaust system leak ahead of converter, or failed catalytic converter.

You can monitor the secondary O2 sensor w/ a OBD2 scanner w/ monitoring capability. If O2 sensor voltage varies form 0 to -0.9 continuously or near those figures then converter is likely faulty.

Check for exhaust system leaks, shake the converter vigorously listening for rattling sound. If it rattles the converter matrix has collapsed and new converter is necessary.

good luck

 

So it is the 02 sensor closest to the engine (upstream)? Also, I do notice occasionally a little miss when cruising, which makes me think it is 02 sensor related, not the cat.

 

I thought the downstream sensor was related to a P0420 but I'm not familiar with V6 Accords. On my ~225K mile four cylinder a new cat fixed it. They just get old.

A related TSB for your car:

tegger(dot)com/hondafaq/p0420/A03-073(dot)pdf

 

Do the easy things first; check for exhaust leaks and shake converter.

I would also reset ECU and see if code code returns.

good luck

 

secondary sensor is the post catalyst sensor. not the upstream one. the secondary sensor is the one specific to monitoring catalyst efficiency. if no other codes exist, chances are good the cat has used it's nine lives

 

*At work* - So can the catalytic convertor just unbolt? That would be an easy check and I'll check for leaks/rattles. If the secondary (downstream) 02 sensor is reading different levels, the leak should occur between the first and second 02 sensor right? It doesn't seem like it would matter after the last 02 sensor.

On a side note, I have went from ~10 codes to one final one.

 

what year is the car? maybe add it to your signature.

 

Right. But even a small leak can trigger a P0420.

I've read about different ways to differentiate between a bad cat and a bad secondary O2 sensor. Aside from using a 2-channel oscilloscope to chart both O2 sensors at the same time, I don't know of any practical way to do so. I guessed that with high mileage my cat was bad and I happened to guess right. Cat's, like primary O2 sensors, fall in the category of long-term maintenance items.

Fortunately with scrap companies paying $100 with free shipping for your old cat, and a new aftermarket cat going for <$200 (49 state), getting a new one isn't all that expensive.

 

Walker has bolt-on replacments.

Exhaust leak can be ahead of forward O2 sensor causing P0420. Apparently, exhaust gas can provide a false reference to the O2 sensor and cause incorrect fuel mixture. Other theory is O2 gets into the exhaust when exhaust pressure is subambient during engine pulse. I don't know who to believe

good luck