2008 Lacrosse code p0420
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2008 Lacrosse code p0420
Hello,
Please help. I have a 2008 Lacrosse 3.8 with only 35K miles.
Parked it in garage all winter. Just took it out and got P0420 code. I just replaced the downstream O2 sensor yesterday and I cleared the code. Today the code came back. I will replace the upstream sensor later this week.
Please don't tell me its my CAT with only 35K miles?
Thanks for your help!
Please help. I have a 2008 Lacrosse 3.8 with only 35K miles.
Parked it in garage all winter. Just took it out and got P0420 code. I just replaced the downstream O2 sensor yesterday and I cleared the code. Today the code came back. I will replace the upstream sensor later this week.
Please don't tell me its my CAT with only 35K miles?
Thanks for your help!
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Joined: Feb 2015
Posts: 6,809
Likes: 1,006
From: Earth
Probably didn't drive much, and when it did, it was probably all slow. Maybe short trips as well. Bad for CAT longevity.
35,000 miles / 17 years = ~2,000 miles per year
If it were an early 2008, it'* be less than that per year.
Was this a garaged, barely-driven occasional-use grocery getter in a big city or small town from the day it was new? Possibly in a Northern state? And the store and the church were only a couple of miles away? I ask this way because there is a demographic that has a prevalence of buying Buick sedans (pre-Chinese-influenced design) brand new and then barely using them and treating them very softly at all times. Not passing judgement, just have seen this countless times. Makes for some good opportunities for nice affordable used cars, but sometimes this is the result.
Catalyst System Efficiency Below Threshold.
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
I know what came out of the combustion process.
I know what the catalytic converter did with the results.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
I agree with carfixer007 ^^^^^^^, this likely won't help
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
The forward O2 sensor read what I expected at all times.
The rearward O2 sensor read what I expected at first, so I know it is good, but then what it read was less and less good as I expected the catalyst to warm up and process gases properly.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
Okay, I won't tell you it'* your CAT with only 35,000 miles.
carfixer007 already did.
I'll tell you that I agree with him about this subject though. Sorry, but I had to find a way around your restriction and still do my part right. Thanks carfixer007 for the help!
So here'* the thing, there are a couple of hokey ways to try to wake this cat up:
They make magic CAT fixer juice. Sometimes it works. Usually it doesn't. If it does, it does so for almost enough mileage for you to clear the codes and sell it with the check engine light off. Almost. Unless you're lucky.
You could try to wake it up by driving on the highway at, let'* say, about 60MPH, and using the transmission selector to get the engine up to about 3,000RPM and then driving that way for like an hour. I've done this to wake up a few cats, but almost never had one remain working "properly" for more than a few hundred miles. Usually the front inch of the catalyst cleans up a little, but when going back to normal driving for a while it has the same problem. My theory is that the rest of the catalyst still doesn't process and keeps the front of the catalyst too cool to process. I've seen the dirty (carbon covered) front of the catalyst clean up after doing stuff like this, but also seen it carbon right back up in a few hundred miles after driving goes back to normal. In my book, this is not worth the trouble. My history has had this work maybe 20% of the time, but almost never last long.
35,000 miles / 17 years = ~2,000 miles per year
If it were an early 2008, it'* be less than that per year.
Was this a garaged, barely-driven occasional-use grocery getter in a big city or small town from the day it was new? Possibly in a Northern state? And the store and the church were only a couple of miles away? I ask this way because there is a demographic that has a prevalence of buying Buick sedans (pre-Chinese-influenced design) brand new and then barely using them and treating them very softly at all times. Not passing judgement, just have seen this countless times. Makes for some good opportunities for nice affordable used cars, but sometimes this is the result.
Catalyst System Efficiency Below Threshold.
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
I know what came out of the combustion process.
I know what the catalytic converter did with the results.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
The forward O2 sensor read what I expected at all times.
The rearward O2 sensor read what I expected at first, so I know it is good, but then what it read was less and less good as I expected the catalyst to warm up and process gases properly.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
Okay, I won't tell you it'* your CAT with only 35,000 miles.
carfixer007 already did.
I'll tell you that I agree with him about this subject though. Sorry, but I had to find a way around your restriction and still do my part right. Thanks carfixer007 for the help!
So here'* the thing, there are a couple of hokey ways to try to wake this cat up:
They make magic CAT fixer juice. Sometimes it works. Usually it doesn't. If it does, it does so for almost enough mileage for you to clear the codes and sell it with the check engine light off. Almost. Unless you're lucky.
You could try to wake it up by driving on the highway at, let'* say, about 60MPH, and using the transmission selector to get the engine up to about 3,000RPM and then driving that way for like an hour. I've done this to wake up a few cats, but almost never had one remain working "properly" for more than a few hundred miles. Usually the front inch of the catalyst cleans up a little, but when going back to normal driving for a while it has the same problem. My theory is that the rest of the catalyst still doesn't process and keeps the front of the catalyst too cool to process. I've seen the dirty (carbon covered) front of the catalyst clean up after doing stuff like this, but also seen it carbon right back up in a few hundred miles after driving goes back to normal. In my book, this is not worth the trouble. My history has had this work maybe 20% of the time, but almost never last long.
Last edited by CathedralCub; Mar 18, 2025 at 02:02 AM. Reason: Mad a couple of critical changes
Thread Starter
Junior Member
Joined: Mar 2025
Posts: 3
Likes: 4
Probably didn't drive much, and when it did, it was probably all slow. Maybe short trips as well. Bad for CAT longevity.
35,000 miles / 17 years = ~2,000 miles per year
If it were an early 2008, it'* be less than that per year.
Was this a garaged, barely-driven occasional-use grocery getter in a big city or small town from the day it was new? Possibly in a Northern state? And the store and the church were only a couple of miles away? I ask this way because there is a demographic that has a prevalence of buying Buick sedans (pre-Chinese-influenced design) brand new and then barely using them and treating them very softly at all times. Not passing judgement, just have seen this countless times. Makes for some good opportunities for nice affordable used cars, but sometimes this is the result.
Catalyst System Efficiency Below Threshold.
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
I know what came out of the combustion process.
I know what the catalytic converter did with the results.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
I agree with carfixer007 ^^^^^^^, this likely won't help
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
The forward O2 sensor read what I expected at all times.
The rearward O2 sensor read what I expected at first, so I know it is good, but then what it read was less and less good as I expected the catalyst to warm up and process gases properly.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
Okay, I won't tell you it'* your CAT with only 35,000 miles.
carfixer007 already did.
I'll tell you that I agree with him about this subject though. Sorry, but I had to find a way around your restriction and still do my part right. Thanks carfixer007 for the help!
So here'* the thing, there are a couple of hokey ways to try to wake this cat up:
They make magic CAT fixer juice. Sometimes it works. Usually it doesn't. If it does, it does so for almost enough mileage for you to clear the codes and sell it with the check engine light off. Almost. Unless you're lucky.
You could try to wake it up by driving on the highway at, let'* say, about 60MPH, and using the transmission selector to get the engine up to about 3,000RPM and then driving that way for like an hour. I've done this to wake up a few cats, but almost never had one remain working "properly" for more than a few hundred miles. Usually the front inch of the catalyst cleans up a little, but when going back to normal driving for a while it has the same problem. My theory is that the rest of the catalyst still doesn't process and keeps the front of the catalyst too cool to process. I've seen the dirty (carbon covered) front of the catalyst clean up after doing stuff like this, but also seen it carbon right back up in a few hundred miles after driving goes back to normal. In my book, this is not worth the trouble. My history has had this work maybe 20% of the time, but almost never last long.
35,000 miles / 17 years = ~2,000 miles per year
If it were an early 2008, it'* be less than that per year.
Was this a garaged, barely-driven occasional-use grocery getter in a big city or small town from the day it was new? Possibly in a Northern state? And the store and the church were only a couple of miles away? I ask this way because there is a demographic that has a prevalence of buying Buick sedans (pre-Chinese-influenced design) brand new and then barely using them and treating them very softly at all times. Not passing judgement, just have seen this countless times. Makes for some good opportunities for nice affordable used cars, but sometimes this is the result.
Catalyst System Efficiency Below Threshold.
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
I know what came out of the combustion process.
I know what the catalytic converter did with the results.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
I agree with carfixer007 ^^^^^^^, this likely won't help
If I read the PCM'* mind, I see this:
I know what is going into the combustion process.
The forward O2 sensor read what I expected at all times.
The rearward O2 sensor read what I expected at first, so I know it is good, but then what it read was less and less good as I expected the catalyst to warm up and process gases properly.
I tried making adjustments to help wake the catalytic converter up, and it didn't work for long enough.
I stored code P0420 and requested the MIL lamp be lit.
Okay, I won't tell you it'* your CAT with only 35,000 miles.
carfixer007 already did.
I'll tell you that I agree with him about this subject though. Sorry, but I had to find a way around your restriction and still do my part right. Thanks carfixer007 for the help!
So here'* the thing, there are a couple of hokey ways to try to wake this cat up:
They make magic CAT fixer juice. Sometimes it works. Usually it doesn't. If it does, it does so for almost enough mileage for you to clear the codes and sell it with the check engine light off. Almost. Unless you're lucky.
You could try to wake it up by driving on the highway at, let'* say, about 60MPH, and using the transmission selector to get the engine up to about 3,000RPM and then driving that way for like an hour. I've done this to wake up a few cats, but almost never had one remain working "properly" for more than a few hundred miles. Usually the front inch of the catalyst cleans up a little, but when going back to normal driving for a while it has the same problem. My theory is that the rest of the catalyst still doesn't process and keeps the front of the catalyst too cool to process. I've seen the dirty (carbon covered) front of the catalyst clean up after doing stuff like this, but also seen it carbon right back up in a few hundred miles after driving goes back to normal. In my book, this is not worth the trouble. My history has had this work maybe 20% of the time, but almost never last long.
A shop down the street says he has a homemade adapter for the downstream O2 that will trick the CAT. I'll try that before I cough up a lung for a $900 CAT.
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True Car Nut
Joined: Dec 2018
Posts: 3,239
Likes: 1,597
From: Flint, MI USA
Do you have inspections? If not, don't worry about it although it could start coming apart and plug the exhaust.
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Joined: Feb 2015
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