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Old 11-27-2018, 05:31 PM   #1
IntheBatCave
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P0420 Catalyst System Efficiency Below Threshold

I'm surprised it took this long for my truck to start throwing codes, but it started giving me a P0420: Catalyst System Efficiency Below Threshold. What are the odds that its actually the cat and not an O2 sensor? The cat is a 3" catco cat that installed eight or nine years ago, and probably only have 10,000 miles on it. The O2 sensors on the other hand are stock.
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Old 11-27-2018, 07:36 PM   #2
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It's the cat. Aftermarket cats just don't have the same amount of reactive material as stock cats. Put a spacer on the post cat sensor and you should be fine. Spacing the sensor away from the exhaust steam tricks the ECM into thinking the cat is working. You didn't hear it from me.
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Old 11-28-2018, 07:27 AM   #3
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Yup, this is what you need (18mmx1.5 thread pitch spark plug non fouler): https://shop.advanceautoparts.com/p/...E&gclsrc=aw.ds

I also know a guy that knows how to make simulators.
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Hot wiring is bad news.
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Old 11-28-2018, 07:12 PM   #4
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Well that is disappointing, but at least it's an "easy" solution for now. Now that I think about it, I wonder if that is why my exhaust smells a little bit. Not of gasoline, but it does have bit more of an odor than I remember it having in the past.

Thanks for the advice and the quick responses.

The post cat O2, is purely for monitoring, not for making any fuel adjustments correct?
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Old 11-28-2018, 07:48 PM   #5
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Quote:
Originally Posted by IntheBatCave View Post
Well that is disappointing, but at least it's an "easy" solution for now. Now that I think about it, I wonder if that is why my exhaust smells a little bit. Not of gasoline, but it does have bit more of an odor than I remember it having in the past.
My Forester stinks a bit because of the cheap post turbo aftermarket converter.

Quote:
Thanks for the advice and the quick responses.

The post cat O2, is purely for monitoring, not for making any fuel adjustments correct?
No problem. There isn't any fueling affect that I'm aware of on GM. Someone once told me some Subarus used it for fine tuning but I could never confirm that.
The ECM monitors both front and rear rich/lean signals and compares them to each other to calculate if the converter is functioning. If you looked at rich/lean O2 signals on a graph it will have a saw-tooth pattern. If both patterns match each other the converter is not properly burning exhaust gasses. If the rear sensor is moved out of the direct stream of exhaust it still sends a signal but at lower amplitude that matches what the ECM wants to see.
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'91 Z70 Bravada lowered 4" 350 Vortec w/TBI, TCI-700r4, KYB's, 9C1 cop car steering gear, rear sway bar, yada yada...
'05 Subaru Legacy wagon (wife's daily)
'04 Subaru Forester XT EJ255 turbo, TBE, lowered 1.5" w/springs
'02 K2500HD single cab 6.0 4L80E 8' plow sander/spreader
Case 580K 4x4 w/chains, 10' power angle plow

To help support this site with a donation click here
My cardomain site
Quote:
"The spirit of resistance to government
is so valuable on certain occasions,
that I wish it to be always kept alive.
It will often be exercised when wrong,
but better so than not to be exercised at all.
I like a little rebellion now and then." -Thomas Jefferson (1743-1826)
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Old 11-28-2018, 08:29 PM   #6
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Yup it's purely for emissions checking. I've made 0-1v 555 timer boards to simulate the rear o2. Along the same lines as what Mitch said, what I've learned is that as long as the ecm sees a switching signal when it polls that sensor it's happy.
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Originally Posted by Teeleton
Hot wiring is bad news.
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