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Old 08-01-2020, 09:02 AM   #1
454cid
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Default P0420 popping up after highway runs sometimes

I've had the P0420 Cat inefficiency code pop up a couple of times now over the course of maybe a month or so. I've read that it's either a dead cat, or a leak in the exhaust. I simply reset it with the scanner.

I think I may have gotten it last fall a time or two as well, but I don't recall for sure. Late last year my mileage dropped off.... maybe when we switched to winter gas, but it was more than I thought it would be.... like 5, 6, or 7 mpg down on the highway. It's back up to the low 30s now....I've hit 34 once. Sometimes I feel like the car isn't accelerating hard enough, and that I'm dragging an anchor, but maybe that's me being in a hurry and expecting too much from a 1.9L 100+ hp engine.

Here's the thing, I was reading that when reset, the code would be back in 150 miles when a test was re-run. I drive that in a day and a half, and I'm only seeing the code after a few weeks...usually after a highway run.

Any ideas? I'm not going to spend money on this. I'm hopping to retire the car this fall. I'm mostly just curious.
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2000 SL2 Automatic (back on the road) 17X,XXX miles.
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Old 08-01-2020, 10:26 AM   #2
Ricketts
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2000 SC1
Default Re: P0420 popping up after highway runs sometimes

Mine does the exact same thing. Everything is good then infrequently, upon deceleration after a long highway trip, I'll pop a 420. I clear it with my scangauge and I'm good again. Mine only pop about once or twice a year though. I might try the laquer thinner trick for potential cat cleaning, but other than that, not going to worry as I'm only hoping to get a couple more winters out of her. Would be curious though if it was a simple cheap fix, I might try it
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Old 08-01-2020, 11:52 AM   #3
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Default Re: P0420 popping up after highway runs sometimes

Are either of you using live-data to check things? There are many things that could cause an intermittent P0420; too many for guessing or parts-swapping, in my opinion.
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Old 08-01-2020, 11:55 AM   #4
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Default Re: P0420 popping up after highway runs sometimes

Quote:
Originally Posted by billr View Post
Are either of you using live-data to check things? There are many things that could cause an intermittent P0420; too many for guessing or parts-swapping, in my opinion.
I wouldn't know what to look for, and since I'm driving I may not have the time to fiddle with the scanner. Time before last, was as I was just getting to work, for instance. My scanner, might store a snap shot of data as the code is set. I could check for that next time.
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Old 08-01-2020, 02:11 PM   #5
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Default Re: P0420 popping up after highway runs sometimes

I understand the difficulty in trying to drive while you watch data; you need to have somebody else drive. Of course, this can be impractical with an intermittent. Look into an ELM327 device and SW that does data-logging.

Honestly, though even with logging it can be a tedious and confusing chore. Hopefully, with just brief looking at live-data you can quickly narrow the search; see flaky readings for sensors like MAP, TP, ECT, IAT, O2, EGR, etc.

Since you are planning on an extreme "parts swap" (replacing car) anyway, just ignoring this code may be most practical for you.
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Old 08-01-2020, 02:34 PM   #6
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Default Re: P0420 popping up after highway runs sometimes

Quote:
Originally Posted by billr View Post
... just ignoring this code may be most practical for you.
That's the plan unless I find out it's some easily fixed thing. The car is getting too rusty underneath for my liking to put money into it.
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Old 08-01-2020, 03:34 PM   #7
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Default Re: P0420 popping up after highway runs sometimes

Get one of these. It pulls the o2 sensor out of the exhaust stream and will turn off the light. You don't have to disconnect the o2 sensor, just thread this in between the pipe and sensor.

https://www.ebay.com/itm/J-Style-O2-...X/113928501357
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Old 08-03-2020, 09:34 AM   #8
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Default Re: P0420 popping up after highway runs sometimes

Been thinking of that, if mine starts doing it more often I'll look into it
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Old 08-12-2020, 08:21 AM   #9
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Default Re: P0420 popping up after highway runs sometimes

I used regular ole spark plug defoulers on my Vue. They run about $7 for two of them.

https://www.amazon.com/Dorman-42009-.../dp/B000CO91FY

I used both of them on the O2 sensor. On one of them, the hole needs to get drilled out so it can screw all the way onto the sensor. Then this screws into the second defouler. You can see this in the photos I took. This was like 4 years ago, never seen a 420 since.

Heres the link on how I did mine:

http://www.saturnfans.com/forums/sho...d.php?t=219738

..
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Last edited by waiter21; 08-12-2020 at 08:35 AM.
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Old 08-12-2020, 09:13 AM   #10
454cid
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Default Re: P0420 popping up after highway runs sometimes

Quote:
Originally Posted by waiter21 View Post
I used regular ole spark plug defoulers on my Vue. They run about $7 for two of them.

https://www.amazon.com/Dorman-42009-.../dp/B000CO91FY

I used both of them on the O2 sensor. On one of them, the hole needs to get drilled out so it can screw all the way onto the sensor. Then this screws into the second defouler. You can see this in the photos I took. This was like 4 years ago, never seen a 420 since.

Heres the link on how I did mine:

http://www.saturnfans.com/forums/sho...d.php?t=219738

..
Thank you. If I pursue a fix, this will likely be it. Right now, I'm messing with my blower motor..... just ordered one. For now I have zip ties putting pressure on the connector that seems to be faulty.

So far the P0420 hasn't come back again.
...
2000 SL2 Automatic (back on the road) 17X,XXX miles.
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Old 09-23-2020, 11:48 PM   #11
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Default Re: P0420 popping up after highway runs sometimes

Quote:
Originally Posted by billr View Post
Are either of you using live-data to check things? There are many things that could cause an intermittent P0420; too many for guessing or parts-swapping, in my opinion.
What info shouldi look at normal live data number etc to compare to ?
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Old 09-24-2020, 12:55 PM   #12
billr
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Default Re: P0420 popping up after highway runs sometimes

I suggest you start a new thread with your specific symptoms, in your own words.

Prime live-data to look at is ECT, MAP and TP. Also, a compression test and fuel pressure check are basic "first tests" for any engine issue.
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Old 09-24-2020, 02:34 PM   #13
454cid
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Default Re: P0420 popping up after highway runs sometimes

Interesting that this came up today..... I'm about to reset the code again.
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