P0420 Code - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 2 (permalink) Old 03-23-2018, 08:13 PM Thread Starter
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P0420 Code

this has been the year of codes for me. evap and 02 sensores and now cat issues. on 2 cars.

i have a 06 2.2 auto cobalt 125k on it. and this code came up p0420 and i ran a ton of tests using the HF scanner. the cat runs at 700 rpm 900 deg at 4k 1200 deg. i did an ir temp and in front of cat like 800 deg back of cat 850 or so, best numbers i can get. i changed the b1s1 sensor last fall. can the rear sensor cause this code because the new front sensor now? engine runs good 28 mpg good pickup and pep. just changed plugs and they did not look bad at all can send pictures of them.

so i have read that air leaking into the system after the tb can cause it. does the cobalt plastic intake start to fail at 125k
can the intake cause coolant to leak into the system ? it is not using oil plugs did not look like oil,coolant, rich, lean, or even worn bad at all


i ordered a sensor to change but for now i did the cheat thing to see if it may work. using the anti fouling adapters and putting the sensor into these and getting it out of the stream,

will the light go off by itself with this cheat or will i need to reset it?

i'm not convinced this will even work but a lot of people are saying it works. yea not fixing the issue but turning off the dam light. i just do not want to start tossing parts at it. i can do the work but getting some ideas helps before the labor starts. if i have to do a cat fine but i do not want to do it and find the new one then fails in a year because of another issue. and i do not have the $$ to let the dealer or repair shops have there go at parts changing repairs.

so if the cobalt at this age and mileage can have intake issues starting to show up i will get a gasket set and or new intake and have a go at it but not wanting to tear into a non issue here either, i will check the coolant tomorrow real close to see it it looks like it may have went down but the oil is still right up there. and like i said the plugs looked clean like a normal running engine. thanks bp


what happened to the other cobalt site with coby7? is he here? thanks.

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post #2 of 2 (permalink) Old 03-24-2018, 04:33 AM
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Coby is here, you're in the right place. 90% of the time a P0420 code is the reasr sensor has failed. I had both of mine fail around 120k km and replaced them and it went another 100k+ km without any more O2 sensor issues.

2009 Ford Escape Limited - 3.0L, 6 speed auto, 4x4, 25/33 city/hwy imp mpg.
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20/25 imp mpg
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02 sensor.coolant , cat , intake , p0420 , sensors

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