CODE P0420 CATALYST SYSTEM LOW EFFICIENCY . I CLEARED THE CODE AND THE CHECK ENGINE LIGHT STAYED OFF FOR ABOUT A MONTH NOW ITS BACK ON SAME CODE . WONDERING IF ANYONE HAS HAD THE SAME ISSUE IF SO HOPING THE FIX WAS SOMETHING LESS EXSPENSIVE THAN THE CATALYITIC CONVERTORS AT WELL OVER A THOUSAND DOLLARS . PLEASE SHARE YOUR EXPERIENCES AND ADVICE .
2015 SONIC CODE P0420
- Thread starter CHEEKS
- Start date
Mine has been on for a couple of years. Replaced the CAT about 6 months from one I bought on ebay that said was good for California. Didn't work. Tried both oxygen sensors last week, didn't work either. Smog due in 1 month. Now I'm thinking I have to go to a repair show. Anyone fix this? I have a 2014 1.8L.
TinySonic10
New member
I had the same issue, ran CatoClean twice and replaced both Upstream and Downstream 02 sensors. Nothing worked. Then I looked under the car and saw the flex pipe just after the forward cat had a small rust hole on it. Ended up replacing the flex pipe section and that fixed the code.
TinySonic10
New member
Car gets a little better gas mileage too.
Is there a rear CAT? Did you fix it yet?
What is the PN? Mine "looks ok" but I have the 1.8L without Flex Fuel.
Not sure what the PN is 95129305, 95129307, or 38759?
How did you clear the code? I only know to disconnect the battery.
TinySonic10
New member
Not sure on what part number, got it from Advanced Auto for like $160. It was the section that bolts to the cat and takes the rear O2 sensor. To clear the code I used a code reader to do it. Disconnecting the battery should also work.
2014SonicLThatch
New member
I am having the same issue - replaced both O2 sensors and still getting the code P0420. The car can be driven everyday for 2 weeks short distances on side roads - and I take it up on the highway one time, get it up to speed and temp - and then when I get to the stoplight after that it comes back on. Sounds like the flex pipe needs to be replaced, but tired of chasing this. Anyone else have success solving P0420?
JTee2017
Well-known member
Take to an auto supply store that will do a free scan that includes pending codes and fuel trim levels. If it is running rich . . . then you know you may have an exhaust leak that is letting air into the system
I see this all the time where someone has an older car with even a slightly noisy exhaust and they do not realize that it is both wasting gasoline and loading up the catalytic converter with unburned fuel.
Daughter had a leaky flex pipe and I told her a number of times to get it fixed.
Finally, after it got even louder than the annoying pfft-pfft-pfft sound, she had it repaired at a muffler shop for $125.
Anyone with a car that's older than 6 or 7 years old should check the exhaust system for small leaks and note ANY exhaust noise that wasn't present when the car was new. Those flex pipes . . flex . . .move from engine vibrations and are exposed to rain, snow, road salt and even internal water vapor when the engine is first started.
The water can sit in there for hours if the car is normally driven on short trips. That internal water condensation is somewhat acidic so will eat the flex pipe and exhaust pipe from the inside out.
I have a similar problem. I just got a P0420 code on my 2015 Chevy Sonic LTZ. I took the attached picture, which I think shows breaks in the pipe to the catalytic converter, which would explain the code. Is there a way to patch the cracks easily, rather than doing a whole converter replacement? Thanks.
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catalytic_converter.jpg
TurtleOverlord
Active member
You could possibly get a shop to weld it for you. I doubt any of the auto shop exhaust patches would work to the flexing nature of the flex pipe.
That is a different section of pipe from the converter though
1.4L:
https://www.wholesalegmpartsonline.com/showAssembly.aspx?ukey_assembly=1829479&ukey_product=10556974
1.8L:
https://www.wholesalegmpartsonline.com/showAssembly.aspx?ukey_assembly=1829478&ukey_product=10556441
SEPT 14 2023: My daughter's 2016 Sonic LT 1.8 Just got the PO 420. It has about 38,000 miles. It is the only code that comes up. I put a shop vac on the tailpipe and went underneath to find an exhaust leak on the front of the flex pipe. Before I mess with the 02 sensors, I am going to fix the leak and hope the 420 goes away.
UPDATE:
Had the flex pipe leak welded and cleared the codes. Two days with no check engine light. So far, so good.
Just FYI, the leak was not easily visible (until using the shop vac) because the flex pipe still looked like it was solidly attached to the end of the CAT pipe. But if you grabbed it, you could slide it back and forth against the pipe.
Still no codes!
UPDATE OCT 25 2023:
PO 420 DID NOT COME BACK! All it was was the small weld around the flex pipe. Even the muffler shop tried to sell me a new CAT because they saw the code on their scanner. I had to go in and point out where the leak was because it is not really noticeable. I only found it using the shop vac on the tailpipe, which is when I could hear it. Try that first just in case. No need to buy an o2 sensor, or a CAT.
UPDATE:
Had the flex pipe leak welded and cleared the codes. Two days with no check engine light. So far, so good.
Just FYI, the leak was not easily visible (until using the shop vac) because the flex pipe still looked like it was solidly attached to the end of the CAT pipe. But if you grabbed it, you could slide it back and forth against the pipe.
Still no codes!
UPDATE OCT 25 2023:
PO 420 DID NOT COME BACK! All it was was the small weld around the flex pipe. Even the muffler shop tried to sell me a new CAT because they saw the code on their scanner. I had to go in and point out where the leak was because it is not really noticeable. I only found it using the shop vac on the tailpipe, which is when I could hear it. Try that first just in case. No need to buy an o2 sensor, or a CAT.
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flex pipe leak_InPixio.jpg
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Emma’s sonic
New member
I know it's been a year but can you tell me if you solved the problem or did you sell the car?
If you read my post up above, it says mine was fixed by welding the flex pipe leak- which was NOT EASILY visible. It is going on two months now, and the check engine light has not come back on. The muffler place read the code and immediately tried to sell me a new catalytic converter, but I had already checked for a leak (read post from sept 14 2023). I had to show him the leak myself because it did not look broken. How many miles does yours have? I did not need an o2 sensor, or even a flex pipe, just a small weld. Just a small exhaust leak there will cause the po420 code. The picture I posted sept 14 shows where my leak was. If I pulled on the flex pipe, it would slide back and forth over the pipe it goes over.
I would check and see if it has a leak there before I did anything else because the shop might not even notice it. I used a shop vac on the tail pipe and I could hear it pretty well. I had to jack up the front to get under it well enough though. Pain in the butt, but it saved me from replacing all that just to have the same problem. Mine only had 37000 miles.
If yours is under 60,000 miles (I think), there is a FEDERAL LAW that says the manufacturer must replace the CAT for emissions standards. That has to be done by the manufacturer (even if you bought from used car dealer). That only applies if the cat is actually bad.
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flex pipe leak_InPixio.jpg
JTee2017
Well-known member
Great job in trouble shooting and using the shop vac to find it.
Thank you for this post and updates !
This is now a good resource for those with future similar issues !
At least . . . . if people would actually take the time to read ALL in a thread or even posts that resolved the issue as you did.
Thanks. I do hope others can save some unnecessary trial and error- and money.
After watching 30 or 40 you-tube videos, I luckily saw one where the guy used a shop vac to test for a leak, which is one thing that could cause the PO420. I would give them credit, but I don't remember the channel.
At least it is a basically free place to start.
I asked the guy at the muffler shop if he had ever seen a car with 37,000 miles with a plugged up cat. He said no, but they were still going to do it. Unless you are a welder, you might have to be on guard at the shop.
Emma’s sonic
New member
That's good news. Thanks and what a great idea to use the shop vac to check for leaks. I use ramps to get under vehicles for oil changes so hopefully I can get the access I need to the flex pipe. Have to locate some oil leaks while I'm there.
Thank you @31003 for posting your troubleshooting technique and how you found the leak. I have a 2013 1.4L (~170k miles) that intermittently sets the P0420 code. I replaced the front pipe ~5years ago because the bellows had a crack in it. I will see if the weld has broken on the newer pipe. There's nothing more aggrivating than having that little yellow engine light come on. 
