P0420 please help!

Tiny
NURSEHEATHER09
  • MEMBER
  • 2009 CHEVROLET MALIBU
  • 69,916 MILES
I just had my code checked and it said that it was a p0420 code and it had another p0420 code with some symbols beside it. How do I know if it is an 02 sensor versus my catalytic converter. I have mechanically inclined family, can they fix it?

Thanks,
nurseheather09
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Wednesday, September 21st, 2011 AT 1:42 AM

24 Replies

Tiny
ASEMASTER6371
  • EXPERT
It is a cat code for ineffiency. The cat most likely has to be replaced. You need to use and original equipment one. If you use an aftermarket cheap unit, the light will come back on and you will be in the same boat.
By the way, you have an 8yr 80k federal warranty. Check with your dealer to see if it can be done for free.

Roy
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Wednesday, September 21st, 2011 AT 1:47 AM
Tiny
NURSEHEATHER09
  • MEMBER
I just got my oil changed about 2 weeks ago and it was overdue. My light just came on this week. Someone told me that it could be my 02 sensor, they said that the oil residue from the overdue change could have messed up one of my 02 sensors. Is this feasible, or was he just shooting me a line? How much could this cost me if it is not covered?
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Wednesday, September 21st, 2011 AT 1:57 AM
Tiny
ASEMASTER6371
  • EXPERT
He was just shooting you a line.
If it is not covered, plan on about 800-1000 for the repair.

Roy
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Wednesday, September 21st, 2011 AT 2:09 AM
Tiny
NURSEHEATHER09
  • MEMBER
Thank you for the information
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Wednesday, September 21st, 2011 AT 3:22 AM
Tiny
ASEMASTER6371
  • EXPERT
Good luck
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Wednesday, September 21st, 2011 AT 3:29 AM
Tiny
MICHAEL BERGER
  • MEMBER
  • 2007 CHEVROLET MALIBU
  • 2.2L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 150,000 MILES
Hi guys I thought you might be able to help me out. I'm getting a p0420 on a 2007 chevy malibu 2.2 liter I replaced the cat with a Dorman and erased the code after about a few miles it went back on so I put another one in reset it and same issue replaced upstream sensor still the same downstream seems fine. Maf sensor seems fine. I checked the data and I noticed that the code was set in open loop? So I reset it again and watched the data and I noticed that when it switched to open loop at idle speed the downstream o2 sensor started jumping up and down and it set the code but in closed loop its steady at around.800a bit high but steady also the code was always set in idle I also have an evap leak but that also shouldn't matter heres the freeze data what's going on!
Abslt tps¬ ¬ 22.7%
Eng speed 789
Baro prs hg 29.8
Calc load.¬ 38.4
Maf¬ lb/m 0.64
Map hg 13.0
Out temp 77¬
Coolant 198¬
IAT 88¬
Ign adv 12.0¬
St ftrm 0.0%
Lt ftrm 5.5%
Veh speed 0
Abs load 28.2%
Fuel system open
Rel tps 8.6%
Thot cmd 12.9%
Abs tps 22.7%
Acc pos D 17.6%
Acc pos E 8.6%
Cat temp 1258¬
Evap purge 0.0%
Cmd eq rat.869

I would greatly appreciate your help thanks!
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • EXPERT
When the vehicle is running in closed loop mode, the upstream oxygen sensor reading should fluctuate. The downstream O2 sensor reading should be fairly steady. Also, one thing that could set the code (among other things) is having the two sensors reading very similarly.

Things I would check would be for any type of exhaust leak, a leaky fuel injector, or a misfire. Based on everything you listed, other sensors seem to be working properly.
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
MICHAEL BERGER
  • MEMBER
Thanks for the response. The downstream sensor seems to be steady it's high but steady regarding misfire there's no misfire count as for an exhaust leak all the gaskets are new. You mentioned a leaky fuel injector, can that set a p0420 code? If it can that would make sense I think with the downstream sensor reading high from a rich condition but what I still don't get is how the code was set in open loop and why the downstream sensor fluctuated in open loop at idoe when it set the code? Thanks guys I really appreciate your help!
.
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
JACOBANDNICKOLAS
  • EXPERT
Start by checking to make sure the fuel pump pressure is within the manufacturer's specs. You may even have a bad fuel pressure regulator.
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
BRIAN08
  • MEMBER
  • 2001 CHEVROLET MALIBU
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 126,340 MILES
I keep getting this code I have replaced my catalytic converter and both o2 sensors. My coolant bottle is leaking around the seals so my mechanic is going to replace it and check the pcm to see if it needs to be replaced so what else could it be?
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
DENNYP
  • EXPERT
Make sure you have a factory replacement converter. I have had many aftermarket converters cause the p0420 code to set.
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
BKSFLYESTPAPI333
  • MEMBER
  • 1998 CHEVROLET MALIBU
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 101,000 MILES
I have already replaced the catalytic converter and I still have the same code coming up with the check engine light. What could the problem be?
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
DOCFIXIT
  • EXPERT
Possible causes for code p0420
Leak in exhaust before cat
HO2S circuit damage check wiring and O2 tight
was cat replaced with OEM?
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
BKSFLYESTPAPI333
  • MEMBER
The cat was replaced by meineke. I'm not sure if they use OEM, I don't think so though.
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Wednesday, May 20th, 2020 AT 1:33 PM (Merged)
Tiny
DOCFIXIT
  • EXPERT
If not GM part code will reset have run into this numerous times
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Wednesday, May 20th, 2020 AT 1:34 PM (Merged)
Tiny
BKSFLYESTPAPI333
  • MEMBER
So it is possible that everything is working fine and the code is just coming up because it's not a GM part? So to be safe I would have to check everything else anyway just to make sure?
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Wednesday, May 20th, 2020 AT 1:34 PM (Merged)
Tiny
DOCFIXIT
  • EXPERT
Did light stay off for a while after cat replaced?
Having the car diagnosticaly scanned would clear up what is going on.
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Wednesday, May 20th, 2020 AT 1:34 PM (Merged)
Tiny
BKSFLYESTPAPI333
  • MEMBER
The light was off for about a week after the cat was replaced. Then it came on and I scanned it with my Actron PocketScan Plus OBD II Code Reader. It came up with P0420, P0420pd. I brought the car back to meinek and they scanned it with their Code reader and said it was probably the O2 Sensor. He erased the code to see if it would come back and it has.
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Wednesday, May 20th, 2020 AT 1:34 PM (Merged)
Tiny
DOCFIXIT
  • EXPERT
Then PCM is seeing a low efficiency in cat. It takes several reads for code to set so reset of PCM works for a while. Did Meniek diagnose the cat replacement?
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Wednesday, May 20th, 2020 AT 1:34 PM (Merged)
Tiny
BKSFLYESTPAPI333
  • MEMBER
Well, when I went to meineke and told them the code I was getting, they lifted the car up, looked at the cat and said the cat needs to be changed. They plugged their code reader and got the same code and still said to change the cat. They didn't test or thoroughly check anything.
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Wednesday, May 20th, 2020 AT 1:34 PM (Merged)

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