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206,000km, 2009 Versa sedan, base model 1.6 manual.

I've been getting code P0420 for a while, and I noticed that my fuel efficiency dropped by about 5mpg.

A while back I also had an issue where it randomly went into limp mode and threw code P0171. I cleaned the mass airflow sensor and those issues resolved themselves, but the P0420 code kept coming back.

My understanding is that I have either a bad O2 sensor or a bad catalytic converter.

a) How can I tell if it's the O2 sensor or the cat?
b) If it is the cat - I estimate my Versa's value around $1k, or around the same cost of having my mechanic put in a new cat. Can I just remove the cat? How would I go about doing that? No emissions test where I live.

No other issues besides the lower fuel economy and the incident where it went into limp mode (so far).

Thanks.
 

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2016 Nissan Versa S (manual transmission)
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You might want to look on ebay for a cheap catalytic converter... You should be able to find one for your Versa for around $120+/-.

Based on the picture it looks like four bolts and the downstream O2 sensor to change it out.
 

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Usually if the catalytic converter is bad you'll smell it, it has that sulfur/rotten egg smell. I'd be more inclined to lean towards the O2 sensor being at fault. The best way to determine would be to get a scan tool that will show you live sensor data and look at your fuel trims.

As to your other point about the converter. I'm not sure what Canada's emissions laws are compared to the US, but we just had a big crackdown from the EPA a few years ago on parts shops selling "race exhausts" that straight pipe the converter. It's a $10,000 fine and an automatic and permanent revokation of your business license if you're caught removing converters.
 

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Get a cheap scan tool that can read live data and I'll walk you through testing it. It's almost certainly that the convertor is shot though. I've never seen it end up being the sensor.

It sucks because it's expensive.

You'll also be able to tell if you fixed the lean code by cleaning the sensor. Data will tell if it's dirty, a vacuum leak, or fuel delivery.

Usually people clean the sensor, think they fixed it, but it's actually a vacuum leak and the light doesn't come on unless it idles long enough, since vacuum leaks are totally hidden while driving.

Just get the cheapest bluetooth dongle with good reviews on amazon and download the torque pro app.


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