Posted: 9/18/2014 9:27:23 AM EDT
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Hi all,
I have a 2002 Toyota Echo that I have had to put some money in, as it was not cared for previously so I think I am at the final issue. I have a CEL code of 0420, the manuals say that is a Exhaust system code. So I've checked the exhaust no leaks, there is no performance loss or otherwise issues to say that the cat is bad, but there are two oxygen sensors, one before the cat and one after, so going off the manual flowchart graphs, the ECU checks the results against the secondary sensor to the primary sensor and trips the CEL code if they match or the seconds results are too close to the first ones. For those with more experience than I, and it being 60 bucks a sensor which one should I replace first?? I am thinking the first one is the way to go, as the car had some misfires due to water in the coils/plug area and that could have dirtied up the sensor, but it could be the secondary one also. what say the almighty arf |
P0420 says that your cat is not working correctly. The sensor before the cat determines AFR. The sensor AFTER the cat is only there to ensure the cat is properly filtering out the harmful gasses. Since there is no performance issue and the code given is usually for rear o2s, If I were to replace one, I'd do the rear o2 sensor. If you're sure the cat is not bad, there are also ways to just doctor it and make it LOOK okay and get rid of the check engine light 
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Quoted:
P0420 says that your cat is not working correctly. The sensor before the cat determines AFR. The sensor AFTER the cat is only there to ensure the cat is properly filtering out the harmful gasses. Since there is no performance issue and the code given is usually for rear o2s, If I were to replace one, I'd do the rear o2 sensor. If you're sure the cat is not bad, there are also ways to just doctor it and make it LOOK okay and get rid of the check engine light The code came on after I got the water out and corrected the hesitation problem I had (more water) so I was going to reset the CEL and see if it returns. As it had misfires in cylinder 1 and 2, so I'm worried that it screwed up the cat. This thing is approaching the value/cost line on fixing things... I am of the mind of putting it up for sale and letting prospective buyers know about the issue and pricing accordingly. |
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There's a chance, but usually in that case you'd have a lack of performance and rattling sound as the cat breaks apart. Or it will start glowing red.
Take a spark plug non fouler and see if the CEL comes back. That's what some of us that don't even run cats do. It's cheap and effective. Here's a good write up |
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Thats a universal code that your cat isn't scrubbing thr exhaust correctly so the O2 sensor after the cat has similar reading as the O2 sensor before the cat. With a correctly working cat, the O2 sensor behind the cat should have much more stable readings when compared to pre-cat O2.
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