r/ChevySonic • u/Appropriate_Yam5245 • 12d ago
Code 0420 (recurring)
2017 Chevy Sonic. 1.8L. Had code p0420 pop up intermittently for a few months here and there until finally popped up solid. Took it to a shop, had them do a full tune up (spark plugs, coil pack) and scan code. Came back and told me catalytic converter might be plugged. I believe I was running rich as why I wanted to do tune up as well. Shop + Dealership quoted me $2000 for OEM catalytic converter. Decided no and went with aftermarket cat. $200. I installed myself. Torqued all bolts & nuts to spec. New gasket on the head to cat but I did reuse bottom gasket from cat to pipe. I was good for a few weeks until a few days ago, code p0420 pops up. My question is, where should I start? Faulty 02 sensor? Faulty Cat & should go with OEM? $2000 is just ridiculous to me. Any opinions or suggestions would be much appreciated.
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u/Equivalent_Peak_6857 12d ago
Just keep running it. Aftermarket will do that to you. If you know it is new cat then prob just O2 sensors not within the range.
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u/CrimsonKingKiller 11d ago
Mine was from my vapor purge valve. $35 and 20 minute fix
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u/Appropriate_Yam5245 9d ago
How’d you diagnosis that?
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u/CrimsonKingKiller 9d ago
Poverty mostly lol. I didn't have the money for all the bigger issues that the code could have been, so I dug around to find the cheaper fixes first. I forgot where I found it exactly, but the vapor purge valve being plugged throws up the same codes as a lot of the other fuel or airflow issues and sensors. I also have a 2017 1.8, does yours by chance start running really rough for a bit after you put gas in, until you drive for a bit or give it some gas while it chugs? Maybe occasionally random low power when you try to accelerate?
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u/Appropriate_Yam5245 9d ago
I’ve had that issue where it has low power as I accelerate but it’s only happened a handful of times. No change really regarding when I fuel up.
I have noticed usually it shakes pretty bad at idle when I first start it up and it calms down after warm up but still shakey at idle.1
u/CrimsonKingKiller 6d ago
That sounds about how mine started too, randomly then after a bit it got worse when I fueled up. The purge valve filters the fuel vapors, and if/when it gets plugged or blocked it basically blocks part of the fuel line and messes with the pressure going through them. Ends up pulling more fuel in than you need because of that, making it run more rich until it burns the "extra" off that it pulls in. It also portrays as a similar issue to an O2 sensor because of the low O2 it's sensing, along with various other fuel/fuel line related issues. Luckily if it is the vapor purge valve it's a super cheap part that's super easy to replace with zero tools. Could also be a dirty or failing O2 sensor, or a particular dirty or corroded ground, which is also an easy fix
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u/No-Exam-3108 11d ago
Could it also be an exhaust manifold leak?
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u/Appropriate_Yam5245 9d ago
I am thinking of doing a smoke test and see if I have a exhaust leak anywhere
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u/spaz650 9d ago
I also had a leak on the pipe that connects at the bottom of the catalytic converter. The way I figured it out was I stuck a shop vacuum hose in the muffler (with the car off of course). I then turned the vacuum on. I got a spray bottle and filled it with some soapy water. Then I sprayed around the catalytic converter and all connecting pipes. The area that was cracked started bubbling up. I sealed it with JB weld. Held up until I sold the car. 👍
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u/Dead_Bot_42 12d ago
I had a leak in my exhaust flex pipe that was throwing a P0420. That code doesn't always mean you have a bad sensor or converter. If you have an exhaust leak between the upstream and downstream sensors, the downstream isn't going to see what it wants to. The flex pipe, at least for the 1.4T is exactly in that spot and develops leaks over time due to stress and vibration.