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Discussion Starter · #1 ·
HELP! I have a 2013 accord which i really like. 120k. I have had the o2 sensors changed twice because after awhile it throws the code which is for the o2 sensor and the mechanic changes it. Have had the spark plugs changed. And now i get this p0420 code for the cat. Took it to another mechanic, he says the cat needs to be changed. Well I know it's for the cat, but doesn't always mean it's that. Could be something that is causing this. Below I will show recent maintenance that has been done. I suspect the quick carbon buildup on o2 sensors have something to do with this code now. I don't know what is causing the o2 sensors to go bad so quickly.
Could it really be the cat? I was quoted at $1000 for a new one with labor. But last thing I want to happen is pay that and the p0420 code doesn't go away...

4/12/21 108,653
- o2 sensor upstream rear: Denso 234-5056
- o2 sensor downstream rear: KAX 250-24785

6/2/21 110,177
  • o2 sensor downstream front: Denso 234-4781
  • Spark plugs: Bosch 9619

11/8/21 115,036
- o2 sensor upstream front: Denso 234-5056

12/27/21 116,122
- o2 sensor upstream rear (2nd time): Denso 234-5056

1/27/22 116,455
- o2 sensor: upstream front (2nd time): NTK 24372

THIS IS WHEN THE P0420 NIGHTMARE STARTED

5/3/22 118,826
- o2 sensor: upstream front (3rd time): Bosch 18083- P0420 STILL HERE
 

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Your mechanic kept on changing upstream O2 sensors? Why? What codes were you getting?

Also, Honda doesn't respond well any non-Denso O2 sensors. That KAX downstream sensor is so cheap that I can't believe it's real, that can't help.

Is the car burning any oil?
 

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2010 Honda Accord (LX)
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I agree with Hudouc. If your car is burning oil that would explain quite a bit.
 
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Discussion Starter · #4 ·
The o2 sensors would be changed when the CEL would come on and the light was for that o2 sensor. I remember one time it being p0137.

the car doesn’t burn any oil, just checked the dipstick it’s still full
 

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P0137 is secondary O2 low voltage. Try to find out what the other codes are, because we can't help without knowing all the information.

I have two theories:
You have a intermittent wiring issues that's causing the primary O2 sensor issues, which in turn is causing your AFR to be off constantly, this resulted the cat going bad.
That KAX sensor is providing bad readings and triggering a false P0420, or providing bad readings and resulted the cat going bad.
 

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The sensor in front of the catalytic converter determines the fuel mixture, the rear sensor monitors the health of the catalytic converter.

You need someone to troubleshoot this and not throw parts at it. Those front A/F sensors are very expensive. The p0420 code is most likely a bad catalytic converter.

Things that kill catalytic converters

1. Bad fuel mixture
2. Burning oil
3. Burning antifreeze
4. Misfires
5. Bad fuel
6. Silicone sealants
7. Physical damage (hard hits to the exhaust system)

I would check your fuel trims and they need to be +/- of 8%. Check the misfire counters. I would adjust the valves, and check compression on all cylinders. Also, be sure the 3rd catalytic converter is not destroyed as well.





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Discussion Starter · #7 ·
P0137 is secondary O2 low voltage. Try to find out what the other codes are, because we can't help without knowing all the information.

I have two theories:
You have a intermittent wiring issues that's causing the primary O2 sensor issues, which in turn is causing your AFR to be off constantly, this resulted the cat going bad.
That KAX sensor is providing bad readings and triggering a false P0420, or providing bad readings and resulted the cat going bad.
I want to say at different times there were p0142, 143, and 147. The o2 sensors for each were changed and the CEL went away. Again these were at different times, not all at the same time. And I hope I am saying the correct codes
 

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I want to say at different times there were p0142, 143, and 147. The o2 sensors for each were changed and the CEL went away. Again these were at different times, not all at the same time. And I hope I am saying the correct codes
Most likely the codes you saw were

P0420
P0430
P0137

All a guess, that is the issue, guessing and parts swapping P0420 and P0430 are not O2 sensor errors. A bad O2 sensor "can" give a bad waveforms and the ECM senses a bad Catalytic converter and set those errors. In general it is most likely a underperforming catalytic converter.


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Thanks. Saw scotty kilmer’s video on youtube about using lacquer thinner. Will try that first and see what happens.
Watch Eric the car guys video and see what you think.



Here is testing for a P0420 code.



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Discussion Starter · #11 ·
Watch Eric the car guys video and see what you think.



Here is testing for a P0420 code.



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Just watched them thank you! I see you have been around this forum for awhile. The v6 has 3 cats from my understanding. The p0420 is for bank 1. How do I know which cat that is and where is it located on the vehicle? Someone told me about using a defouler to make my problem go away.
 

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The code is for Bank 1, that is the catalytic converter nearest the firewall. The third catalytic converter has no sensors. The normal failure of the third catalytic converter is the upstream catalytic converters fail and clog the 3rd catalytic converter.

A defouler adapter on the secondary O2 sensor changes the waveform to the ECM and gives incorrect data and "fools" the ECM to not set the P0420. It does not fix the issue.

I will say this, quite a number of people have had a primary Catalytic converter fail and the engine ingests pieces of catalytic converter and bends the valves. That is an extreme case but it has happened.

You need to go through the troubleshooting to determine what is happening. If your primary O2 sensor is working correctly or not.

A replacement Catalytic converter is about $350, as long as you do not need a CARB compliant converter.

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