P0420 code

[SKnight]

Good afternoon!

I have a 2015 1.4L Turbo. About a month ago the check engine light came on and it was p0420. Took it in to two shops, both told me to replace the Catalytic. So I did. Light was off for less than a day and came back with the same code. Took it back to the shop, they replaced my Catalytic again under warranty. They also replaced my o2 sensor, spark plugs and ignition coil pack. The light would not go off. We finally found a small crack in the exhaust flex pipe. Replaced that, light was off for 2 days and came back, same code.

What could possibly be wrong with it? I only owe $900 still and have spent close to $3k in repairs. I'm out of money at this point and need it to pass inspection next month. No one is able to get it back in until next week, meaning I'm driving around a car with an unknown issue as I can't afford Ubers every day.

 

[melk]

Funny enough I just got the same code today. Was hoping it was just the upstream O2 sensor.

 

[SKnight]

Funny enough I just got the same code today. Was hoping it was just the upstream O2 sensor.

Did you figure yours out yet? I'm still in the dark and need an inspection soon. At this point, I'd rather just replace my car.

 

[Blake1111]

I don't know how much research you did, but here are some causes that could lead to that code.

• Worn or internally failing catalytic converter (Most Common)
• Faulty oxygen sensor
• Faulty air-fuel sensor
• A leak in the exhaust system
• Misfire (the root cause of converter failure)
• Rich or Lean air-fuel ratio (the root cause of converter failure)
• Leaded Gasoline (the root cause of converter failure)

Usually, the mechanic is supposed to actually find the root cause of the issue, not assume that it means what it typically does and just replace that part. I would speak with someone about this again and make sure they know that you want them to examine all of these possibilities. Hope this helps.

 

[SKnight]

I don't know how much research you did, but here are some causes that could lead to that code.

• Worn or internally failing catalytic converter (Most Common)
• Faulty oxygen sensor
• Faulty air-fuel sensor
• A leak in the exhaust system
• Misfire (the root cause of converter failure)
• Rich or Lean air-fuel ratio (the root cause of converter failure)
• Leaded Gasoline (the root cause of converter failure)

Usually, the mechanic is supposed to actually find the root cause of the issue, not assume that it's something that it usually is in most cases. Hope this helps.

Shouldn't someone have figured out by now what it is? I've been to two shops and a dealership. We've fixed 4 things on that list already. The air-fuel sensor and ratios are the only thing left that I haven't heard anyone say anything to me about. Whether it's been checked or not, I'm out of money to spend on diagnostic fees and repairs

 

[Blake1111]

I don't know how much research you did, but here are some causes that could lead to that code.

• Worn or internally failing catalytic converter (Most Common)
• Faulty oxygen sensor
• Faulty air-fuel sensor
• A leak in the exhaust system
• Misfire (the root cause of converter failure)
• Rich or Lean air-fuel ratio (the root cause of converter failure)
• Leaded Gasoline (the root cause of converter failure)

Usually, the mechanic is supposed to actually find the root cause of the issue, not assume that it means what it typically does and just replace that part. I would speak with someone about this again and make sure they know that you want them to examine all of these possibilities. Hope this helps.

Shouldn't someone have figured out by now what it is? I've been to two shops and a dealership. We've fixed 4 things on that list already. The air-fuel sensor and ratios are the only thing left that I haven't heard anyone say anything to me about. Whether it's been checked or not, I'm out of money to spend on diagnostic fees and repairs

Ya, they should have figured it out already, but sometimes the issue is hidden and complicated. You said you are out of money, but if you knew someone with an aftermarket tuner like HP tuner, you could have them hook that to your OBD2 port and scan your car. It will show you all of the air fuel ratios and all that highly technical stuff. I purchased one from ZZP when I was having my sonic tuned. It's like $450, but I think it's worth it. Then again, I am a car enthusiast. lol

 

[Blake1111]

Your other option is finding someone online that does this type of stuff as a hobby, and tell them your situation? like craigslist maybe

 

[SKnight]

Ya, they should have figured it out already, but sometimes the issue is hidden and complicated. You said you are out of money, but if you knew someone with an aftermarket tuner like HP tuner, you could have them hook that to your OBD2 port and scan your car. It will show you all of the air fuel ratios and all that highly technical stuff. I purchased one from ZZP when I was having my sonic tuned. It's like $450, but I think it's worth it. Then again, I am a car enthusiast. lol

I am not lol. I just need the car to get me from point a to point b without breaking. The only reason I haven't straight up replaced my Sonic yet is because I haven't found anything decent in my price range. It's not the first time we have had to pour a couple grand into repairs on it.

 

[Blake1111]

I am not lol. I just need the car to get me from point a to point b without breaking. The only reason I haven't straight up replaced my Sonic yet is because I haven't found anything decent in my price range. It's not the first time we have had to pour a couple grand into repairs on it.

Are the symptoms obvious to you by chance? If so, did you notice a change in them after the parts were installed?

 

[SKnight]

Are the symptoms obvious to you by chance? If so, did you notice a change in them after the parts were installed?

The car was misfiring before. We replaced the ignition coil pack and spark plugs, and I've had no symptoms since. I've driven close to 700 miles since and it drives totally fine. Even had other people drive it and no one noticed anything out of the ordinary. My gas efficiency is back up, it accelerates normally, doesn't have a stutter to it on the highway anymore.

 

[Blake1111]

hmmm, ok...

Well, I believe I actually had this code come up while I was installing a bunch of aftermarket parts and having my sonic tuned. In my case it was a false code reading. usually this means an exhaust leak, but you said they already found a leak... so... Maybe it's another leak that they didn't find? There are at home methods to check that yourself.

I think a failing turbo, or anything else along those lines, that could be producing toxic fumes that make it to the O2 sensors before and after the converter.

Only other thing that comes to mind is maybe your ECU (cars computer) is malfunctioning. Other than that I don't know.

 

[Blake1111]

And instead of replacing the sensors again, just in case you do try, you can always have them checked for malfunction. I wish you luck. This stuff is very frustrating, trust me, I know lol. When I was going crazy trying to make my sonic faster and louder, all kinds of codes would pop up, and I went through hell trying to fix them.

 

[SKnight]

T

And instead of replacing the sensors again, just in case you do try, you can always have them checked for malfunction. I wish you luck. This stuff is very frustrating, trust me, I know lol. When I was going crazy trying to make my sonic faster and louder, all kinds of codes would pop up, and I went through hell trying to fix them.

Thanks! If I can get into a dealership without them asking me to leave the car for several days, I'll ask them to check the things you suggested!!

 

[SuperS13]

December 2021, I had a p0420 and p0171 code and I have a 1.8L the front pipe has a flex section that has constant strain and is known for failure as well as the rear o2 sensor. I have a zzp front pipe on and the flex section started to fray from engine movement and exhaust vibration, so I replaced just the flex section with another stainless steel flex section and tigged it in and looked at my data logging from my bnr tuner and labtop and noticed the rear o2 voltage was all over the place(extra air from flex section) and made the o2 sensor faulty. Both codes are related. Replace the flex pipe and rear o2 sensor and reset the cel and you should be fine. The p0420 code just means air is backtracking up to your primary catalyst and leaning out the bank 1 and bank o2 sensors

 

[SKnight]

update!

I changed the air filter, myself, because it needed done. And two days later... the light turned off. Did I do all that and spend a small fortune for nothing?

 

[SuperS13]

update!

I changed the air filter, myself, because it needed done. And two days later... the light turned off. Did I do all that and spend a small fortune for nothing?

No...its was a good thing you did that, but I forgot to say as well the mass air flow sensor behind the intake box can get gummed up and fail...just for routine maintenance I'd use a bit of mass airflow sensor cleaner to clean the maf sensor, but yeah air filter is general maintenance.