# P0420 Engine Code (Spent $1900 still isn't fixed)



## JLL (Sep 12, 2017)

Take it back to the shop and let the figure out how to fix it.


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## BDCCruze (Jul 12, 2017)

Even if the converter was replaced, if the root cause was left unfixed, then the issue will reappear. Also, the converter put on may of been a used one or possibly be a junk brand.

It sounds like you need to have a pressure check done on both cats to verify they are not clogged.

And here's a tip for you as well. When you take it to a shop you need them to guarantee the diagnosis and fix. If they just want to try parts, that's called the 'parts cannon' when they don't really know what's wrong, they just want to start putting parts on, at your expense. You may need to get a second opinion, but you will likely have to pay a diagnosis charge, likely around $100-120 if you don't have the work done there.


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## BuckeyeBreeze (Mar 17, 2021)

BDCCruze said:


> Even if the converter was replaced, if the root cause was left unfixed, then the issue will reappear. Also, the converter put on may of been a used one or possibly be a junk brand.
> 
> It sounds like you need to have a pressure check done on both cats to verify they are not clogged.
> 
> And here's a tip for you as well. When you take it to a shop you need them to guarantee the diagnosis and fix. If they just want to try parts, that's called the 'parts cannon' when they don't really know what's wrong, they just want to start putting parts on, at your expense. You may need to get a second opinion, but you will likely have to pay a diagnosis charge, likely around $100-120 if you don't have the work done there.


Could it be that the leak in the turbo has caused a long term issue with the cat? So even though the cat was replaced before, the problem has reemerged? I will be calling in the morning to have them do the pressure check. I saw a video by Scotty Kilmer suggesting the same thing. So far they have only charged me when they did work not when they have looked at the car. However, I will be much more firm moving forward with this.


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## BuckeyeBreeze (Mar 17, 2021)

JLL said:


> Take it back to the shop and let the figure out how to fix it.


Yes, but I want to know more for myself. This has been a two and a half week process.


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## JLL (Sep 12, 2017)

BuckeyeBreeze said:


> Yes, but I want to know more for myself. This has been a two and a half week process.


It's almost impossible to accurately diagnose over the internet unless it's a common issue.


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## BuckeyeBreeze (Mar 17, 2021)

JLL said:


> It's almost impossible to accurately diagnose over the internet unless it's a common issue.


Okay thanks


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## Noiitekk (Feb 28, 2017)

That code is almost always either an oxygen sensor, catalytic converter, fuel cap or exhaust leak.


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## Blasirl (Mar 31, 2015)

BuckeyeBreeze said:


> So I have a 2011 Chevy Cruze 1.4l Turbo that I purchased used in a private sale. After leaving the title office the CEL came on, got the code read at auto zone and it is the P0420. The catalytic converter had already been replaced by the previous owner less than a year ago. I took it to the shop and I had an exhaust leak near the flex pipe, they replaced a seal down there and the leak was fixed. The light came back on after that. I then thought that perhaps it could be cause by bad oil so I had an oil change done, it was 3k over and black, and the light came back on. Throughout this whole thing, I had a consistent smell in the cab of my car that was driving me crazy. I had to drive the car with the windows down. I learned that it was burning oil from a leak in the turbo. I spent $1650 to completely replace the turbo as there was a crack in the turbo and in the oil lead line. They said they replaced the whole system and they told me that the code is probably caused by the converter sucking that burnt oil air into it and messing up its efficiency. A few hours after picking up the car, the light came back on. I live in one of the 7 counties in Ohio that require an echeck but with that CEL on I cannot pass echeck. Any ideas what could be going on here?


A code *P0420* may mean that one or more of the following has happened: 
Leaded fuel was used where unleaded was called for (unlikely) 
A damaged or failed oxygen / O2 sensor 
Downstream oxygen sensor (HO2S) wiring damaged or connected improperly 
The engine coolant temperature sensor is not working properly 
Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe 
Failed or underperforming catalytic converter (likely) 
Retarded spark timing 
The oxygen sensors in front and behind the converter are reporting too similar of readings 
Leaking fuel injector or high fuel pressure 
Cylinder misfire 
Oil contamination
Read more at: P0420 Catalyst System Efficiency Below Threshold


Take it back to the mechanic and tell them to fix it. Ask for the turbo they took off back, although it is probably too late. The turbos having a crack in them is nothing new and most of the time it is not an issue. What did you mean when you said "oil lead line"? 

You really should spend some time reading through the forum. Most of your issues have been covered here a 1000 times. Use the search bar and keep refining your search.


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