what I should have done about P0420
10-26-2014, 06:15 PM
#1
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Join Date: 01-23-2009
Location: Fredericksburg,VA
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what I should have done about P0420
This is a post mortem of my P0420 fix.
1. fix any other codes
2. graph O2 sensor outputs
#1 O2 sensor should vary widely and frequently betweem .1V and .9V. #2 O2 sensor should show a fairly straight line something above .5V.
If either is straight below .5V one sensor is bad, if #2 mimics #1 bad catalytic converter.
3. perform cylinder compression test, then leakdown test if indicated
Nominal is about 175 PSI. Mine were #1 175, #2 175, #3 180, #4 200. No need for leakdown.
However, the high #4 indicates carbon buildup in that cylinder. Carbon buildup is caused by too much fuel in the combustion mixture. This indicates a leaking injector.
The way the OBDII Catalyst Monitor works is: when the proper conditions are encountered in a drive cycle the air/fuel mixture is momentarily forced rich then immediately momentarily forced lean. The computer looks for how fast the #2 O2 sensor reacts to the change. If it is slower than expected a P0420 DTC is set.
If an injector is leaking the switch from rich to lean will be too slow.
After I spent lots of money on O2 sensors and catalytic converter and ECU, it turns out that 4 new injectors cured the problem.
None of the diagnostic articles I saw mentioned the compression test! I did it after I replaced the injectors on general principles. I got 4 "Genuine GM" injectors from a Chinese vendor for $88 shipped (got them in 6 days!). It would have cost me at least $80 to have them tested and maybe cleaned.
I always have noticed a lot of injector clickety-clack, it is gone now.
1. fix any other codes
2. graph O2 sensor outputs
#1 O2 sensor should vary widely and frequently betweem .1V and .9V. #2 O2 sensor should show a fairly straight line something above .5V.
If either is straight below .5V one sensor is bad, if #2 mimics #1 bad catalytic converter.
3. perform cylinder compression test, then leakdown test if indicated
Nominal is about 175 PSI. Mine were #1 175, #2 175, #3 180, #4 200. No need for leakdown.
However, the high #4 indicates carbon buildup in that cylinder. Carbon buildup is caused by too much fuel in the combustion mixture. This indicates a leaking injector.
The way the OBDII Catalyst Monitor works is: when the proper conditions are encountered in a drive cycle the air/fuel mixture is momentarily forced rich then immediately momentarily forced lean. The computer looks for how fast the #2 O2 sensor reacts to the change. If it is slower than expected a P0420 DTC is set.
If an injector is leaking the switch from rich to lean will be too slow.
After I spent lots of money on O2 sensors and catalytic converter and ECU, it turns out that 4 new injectors cured the problem.
None of the diagnostic articles I saw mentioned the compression test! I did it after I replaced the injectors on general principles. I got 4 "Genuine GM" injectors from a Chinese vendor for $88 shipped (got them in 6 days!). It would have cost me at least $80 to have them tested and maybe cleaned.
I always have noticed a lot of injector clickety-clack, it is gone now.
04-26-2018, 11:22 AM
#4
Join Date: 04-26-2018
Location: las vegas
Posts: 2
Getting the P0420 code
Running Torque Pro OBDII comes up with this:
MID:$21 TID:$a0
- Catalyst Monitor Bank 1
Max: 0.99939 Min: 0.49971
Test result value: 0.10986
FAIL
Trying to help my father in-law fix his code, is worth replacing the middle sensor?
Running Torque Pro OBDII comes up with this:
MID:$21 TID:$a0
- Catalyst Monitor Bank 1
Max: 0.99939 Min: 0.49971
Test result value: 0.10986
FAIL
Trying to help my father in-law fix his code, is worth replacing the middle sensor?
04-26-2018, 01:16 PM
#5
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Join Date: 01-23-2009
Location: Fredericksburg,VA
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It is really most likely he needs a new cat. Look on eBay they are getting cheaper. It is a 5 bolt repair if you get a complete downpipe + replace the sensors.
Addendum to my OP: That only lasted a few days, I traded the car before I ever figured out what was going on, even a new cat wouldn't fix mine. That seems to be rare.
Addendum to my OP: That only lasted a few days, I traded the car before I ever figured out what was going on, even a new cat wouldn't fix mine. That seems to be rare.
04-26-2018, 02:33 PM
#6
Join Date: 04-26-2018
Location: las vegas
Posts: 2
It is really most likely he needs a new cat. Look on eBay they are getting cheaper. It is a 5 bolt repair if you get a complete downpipe + replace the sensors.
Addendum to my OP: That only lasted a few days, I traded the car before I ever figured out what was going on, even a new cat wouldn't fix mine. That seems to be rare.
Addendum to my OP: That only lasted a few days, I traded the car before I ever figured out what was going on, even a new cat wouldn't fix mine. That seems to be rare.
Is it two or 3 sensors? I am looking at eBay right now and will probably buy something today
Also, there is a listing for a 2.4l cat, but don't they use the same one in both 2.2l and 2.4l models?
04-26-2018, 04:00 PM
#7
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2 sensors, I believe the pipe is larger on 2.4L.
A new cat will be under $200 + labor + sensors.
If I was trading I'd let them worry about it unless they deduct "msrp" for the repair. MSRP on a cat is in excess of $600.
BTW, you did not say what state he is in; California, NY and a couple others have special cats.
A new cat will be under $200 + labor + sensors.
If I was trading I'd let them worry about it unless they deduct "msrp" for the repair. MSRP on a cat is in excess of $600.
BTW, you did not say what state he is in; California, NY and a couple others have special cats.
05-20-2020, 07:52 AM
#8
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Join Date: 01-17-2019
Location: 34652
Posts: 42
Ok So I Took The Car into Check on PO420 Code They said it was Most likely Aftermarket Cat... And shouldn't Worry about it... So I saw a post on here about taking a 100-mile ride is that an option ??
they Also, told me 02 sensors were working as They Should Any Suggestions?
they Also, told me 02 sensors were working as They Should Any Suggestions?
05-20-2020, 08:15 AM
#9
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Join Date: 01-23-2009
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The O2 sensors are working because they are showing the code. They always blame after market cats first. My new injectors only cured it for a month.
There is a TSB that advises getting a new cat good and hot before giving up.
The after market cat I got had really poor welding done on it ; it had many air leaks. I never plugged all of them. Not sure if that makes a difference.
There is a TSB that advises getting a new cat good and hot before giving up.
The after market cat I got had really poor welding done on it ; it had many air leaks. I never plugged all of them. Not sure if that makes a difference.
06-02-2020, 06:20 PM
#10
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Join Date: 01-17-2019
Location: 34652
Posts: 42
So I'm Back I went through everything and the last word was PO420 A mech at a shop I brought it to said Everything was working as it should. The Code is because of the aftermarket Cat So Now Do I Live with it? Is it a problem for the Engine? I don't want to get another cat cant afford it. Any recommendations on what to do next ?
