Do I need a new catalytic converter or just a new oxygen sensor?

Tiny
GADFLY87
  • MEMBER
  • 2007 HONDA FIT
  • 1.5L
  • 4 CYL
  • FWD
  • AUTOMATIC
  • 151,000 MILES
I need some help analyzing some data I collected from my 2007 Honda fit using my OBDII scan tool and the Torque app (See images). For some background, my car came up with a P0420 code (Catalyst System Efficiency Below Threshold) after my car experienced a series of massive misfires. To fix the misfiring problem, I replaced all four spark plugs, all four coil packs, did a valve adjustment and replaced the upstream oxygen sensor (the sensor ended up being the main culprit).

Anyway, the problem I am having now is a P0420 code.

I am trying to determine whether a new converter is necessary. If you look at the images below, you will see that at idle, 1500 rpm and 2500 rpm the upstream O2 sensor is describing a nice waveform that averages about 0.45 volts. In the same instances, the downstream sensor is showing a relatively flat line around 1 volt. This is indicating a properly functioning catalyst.

On the other hand, the last two images seem to suggest a broken downstream sensor. I recorded data on engine start up. The strange thing about this data is that the downstream sensor is showing a flat line even though the converter is less than 300 degrees. The downstream sensor should mimic the upstream sensor when the converter is at these low temperatures. Also, when the vehicle is in stop and go traffic, the downstream sensor is showing a lot of noise. I'm not sure what explains the pattern in the last image. The downstream appears to be moving in an inverse fashion with respect to the upstream sensor.

Any thoughts on this? Thanks!
Monday, July 27th, 2015 AT 6:59 PM

6 Replies

Tiny
HMAC300
  • MECHANIC
  • 48,600 POSTS
Unless there is an air leak in the exhaust it's more than likely the converter.
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Tuesday, July 28th, 2015 AT 6:02 AM
Tiny
GADFLY87
  • MEMBER
  • 3 POSTS
Thanks for the reply!

I'm curious about why you think it's the converter? What about the data is leading you to that conclusion?

According to the wave patterns (except in stop and go), the converter is doing its job. Am I wrong about that? Can you tell me why?
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Tuesday, July 28th, 2015 AT 7:07 AM
Tiny
HMAC300
  • MECHANIC
  • 48,600 POSTS
Because you didn't list any of the codes listed in pic after replacing O2 sensor it can also be poor quality fuel like 15%alcohol instead of 10%
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Tuesday, July 28th, 2015 AT 8:45 AM
Tiny
HMAC300
  • MECHANIC
  • 48,600 POSTS
It also has to be driven at a steady speed for this test.
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Tuesday, July 28th, 2015 AT 8:46 AM
Tiny
GADFLY87
  • MEMBER
  • 3 POSTS
So, before I replaced the upstream O2 sensor, I had misfire codes for all four cylinders (but only misfire codes).

After replacing the upstream O2, the trouble codes disappeared. However, days later, my car threw the P0420. That's the only code I have right now.

Is bad fuel a possible cause of P0420? If so, perhaps I should clear the code now that the car has burned through at least 2 tanks of gas.
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Tuesday, July 28th, 2015 AT 9:13 AM
Tiny
HMAC300
  • MECHANIC
  • 48,600 POSTS
Just 1 tank will do it not 2 see pic for testing to see if it's a converter or not. I think it is with your mileage.
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Tuesday, July 28th, 2015 AT 12:10 PM

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