P0420 and P0106 - related?

The O2 sensor after the cat does not affect performance or gas mileage, it is only there for emisions to measure the cats performance. It does not mean it is dead or necessarily going it just means that on that day it was out of spec. After resetting the CEL it may be months before it might see another trip. In forced induction cars we run richer than normal and sometimes get this CEL code, but it can be reset and go your merry way for a long time.

We also will trick the system by adding a resistor and capacitor to the rear sensor to give the ECU a constant good signal. No CEL lights come on and passes emissions. If we do not see a degradation in performance due to the cat performance then save a few dollars and leave it along.

Most likely your gas consumption was a different day of driving, not cat related. If your cat was really bad you would see restriction in the exhaust and seat of the pants loss in power.

Gary

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Originally Posted by trackdog

The O2 sensor after the cat does not affect performance or gas mileage, it is only there for emisions to measure the cats performance. It does not mean it is dead or necessarily going it just means that on that day it was out of spec. After resetting the CEL it may be months before it might see another trip. In forced induction cars we run richer than normal and sometimes get this CEL code, but it can be reset and go your merry way for a long time.

We also will trick the system by adding a resistor and capacitor to the rear sensor to give the ECU a constant good signal. No CEL lights come on and passes emissions. If we do not see a degradation in performance due to the cat performance then save a few dollars and leave it along.

Most likely your gas consumption was a different day of driving, not cat related. If your cat was really bad you would see restriction in the exhaust and seat of the pants loss in power.

Gary

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+1 I've had a cat go bad. You will definately know when this happens, by the sound of rushing air, and very poor performance, and possibly the sound of marbles in your engine.

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Originally Posted by TurboDuane

+1 I've had a cat go bad. You will definately know when this happens, by the sound of rushing air, and very poor performance, and possibly the sound of marbles in your engine.

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That would explain why I've been getting some pinging, lately. I thought it was just bad gas or something like that.

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Originally Posted by Walsted

Yes, my life is that boring.

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:rolleyes:

Walstad
even if the code is held in the pc it will show as a code that has cleared on an inspection.
If the CEL is not coming on and you are getting a little pinging, it very well may have been a gas problem.
A bad cat usually will darg you down in performance badly.
The one I had that went bad was noticed when I gave a lot of pedal push trying to get in traffic it was a chug chug chug and top speed about 40 mph
I would not worry to much about codes that are cleared or that you have seen stored and cleared
Get all of them cleared and then give it a few days and test it again and see if they come back .
possibly someone could have removed the light from your cel so it cant come on so be sure it is working
Do let us know if it turns out to be a cat I for one am curious to know

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Originally Posted by trackdog

The O2 sensor after the cat does not affect performance or gas mileage, it is only there for emisions to measure the cats performance.

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I figured the decrease in gas mileage was due to the P0106. If it doesn't go back up now that the vac port and electrical connectors are cleaned, I'll start looking for something else.

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Originally Posted by doc

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Get all of them cleared and then give it a few days and test it again and see if they come back .

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The P0420 kept coming back.

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...possibly someone could have removed the light from your cel so it cant come on so be sure it is working.

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It works

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...Do let us know if it turns out to be a cat I for one am curious to know

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I replaced the cat and the codes haven't come back after 50 miles. The old cat was one of those cheap "high flow" versions, the new one is the Magnaflow 99204HM, and the exhaust seems quieter, now.

Hope this helps someone.