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I have a 2005 Buick Lacrosse CXL, 3800 series 3. I have been dealing with P0420 since April. I replace a single part and reset my CEL each time. So far i have replaced both 02 sensors, the catalytic (factory style with new gaskets), the gas cap, and most recently the canister solenoid (located near the fuel filler https://www.rockauto.com/en/moreinfo.php?pk=5411070&cc=1430915&jsn=1).

Should I keep moving up the chain? Replace the under hood evap solenoid? (https://www.rockauto.com/en/moreinfo.php?pk=437883&cc=1430915&jsn=775)

I put links to the parts I'm referencing. My wife is sick of me throwing money at the car.
 

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1998 GTP coupe. All stock and rust free. That will change soon.
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P0420 is post cat O2 sensor. Either the sensor is bad or the cat is bad. Why are you chasing EVAP parts?

Find a friend with a tuner and change the code to "not reporting"
 

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I guess try replacing the under hood the under hood solenoid..... it looks like you have already tried everything else. In reading the flow charts, there is mention of the cat not being efficient. I haven't run across that problem personally but it has been said after market cats may not be as efficient as OEM STYLES are.

If I were in your shoes, I would try getting a good diagnostic scan using a professional scanner........SNAP ON....OTC.... OR????

Another option could be to contact DYNO TUNE MOTOR SPORTS and talk to DYNO BRIAN to get that PO 420 code removed. Reading you post, I'm wondering if the cat you installed isn't working properly...??? Maybe take a close look at the O2's wiring just in case there's a short or open circuit somewhere. Going out on a limb, is it possible the cat just isn't working as it should??? It has been said that after market cats are not sufficient enough. I can't personally verify that but that was mentioned in the flow charts.

It would be nice to see live data using a DIAGNOSTIC SCANNER that shows LIVE data. with these you can watch the actual numbers which can help pin point this culprit.

Get back if you find anything else....... Jake

PS, when driving or idling, what are the O2 NUMBERS?? Generally they will go up and down from the .450mv level...... is there any damage to the O2 sensor wires? .......If you hold the engine at 1500-2000 rpm and go back to idle(this heats up the cat), do the numbers get better and oscillate up and down from the base .450mv? One other thing I just thought about is, have you tried cleaning the MAF SENSOR? If that gets too dirty, it can cause issues also. I use a Q tip and brake cleaner to get them squeaky clean......
 

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Discussion Starter #4
As previously stated, I've installed new o2 sensors, new gas cap, new factory style bolt on catalytic with new gaskets, and the evap can in the back of the car.
 

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I am using an OTC Tools 3208 OBD II & ABS Scan Tool with Enhanced Engine and Transmission Code Reader. Will need to have my wife drive while I monitor the display this weekend. I believe my problems are happening between 75-100 miles after resetting CEL. I do intend to purchase this item an install.(https://www.rockauto.com/en/moreinfo.php?pk=437883&cc=1430915&jsn=453 )

What I failed to note previously is that my 2005 CXL has about 204,xxx miles right now. So wear & age on these parts I expect. Just going nuts tracking down any possible related issue to this generic code. If I get a CEL after I replace the under hood purge solenoid, I'll need to read up on performing the drive cycle so that the EVAP cycle can complete.
 

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How many O2 sensors does it have ? How many have you replaced ?

Are these 2 or 4 wire sensors ? Have you checked the wiring ? Are they outputting a signal ?

Do you still have the old cat ?

Best of luck , :)

Wyr
God bless
 

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My 2002 LeSabre had the 0420 code, replaced both O2 sensors, didn't fix it.
I did the spark plug no-fouler trick, been a year now, still good.
 

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In my case, the Ignition Control Module tanked. I lost spark in cylinders 2 and 5. I was on the way to work in the morning. I continued to work and then home at the end of the day. Total of 75 miles. All the time (while on the throttle) the CEL was flashing. While stopped at a light, the light was solid. NEVER drive with a flashing CEL. The honeycomb effectively was coated with raw gas that rendered the CAT ineffective. No third party made a CAT for my engine family (California emissions, even though I’m in NY), and the mechanic I was dealing with would ONLY install a factory CAT. After CAT was replaced, car was ready for inspection 2 days later. That was in Feb this year and 10K miles later, no issues whatsoever. Unfortunately, I know too much about this issue cause I lived it for 2 months.
 

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Oh yea... I also discovered the car is an SULEV vehicle - go figure! If it got 45 MPG (I’m at 23), the car would be eligible for a NY Clean Air tag which would give me HOV privileges.
 

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Discussion Starter #12
As previously stated, I've installed new o2 sensors, new gas cap, new factory style bolt on catalytic with new gaskets, and the evap can in the back of the car.

I am installing a new evap solenoid tonight, and performing an evap drive cycle tomorrow.
 

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Last night I replaced the Vapor Canister Purge Valve (https://www.rockauto.com/en/moreinfo.php?pk=437883&cc=1430915&jsn=1) and reset my monitors again with my scan tool.

I may end up just doing the "spark plug no fowlers" trick and be done with this if the P0420 comes back.

I was doing these EVAP systems replacements because I had got a code on them while doing these emission parts swaps but that note has gone away in more recent scans.
 

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P0420 = Cat efficiency is under threshold. As was explained to me, your cat might be at 94%, but your still under the threshold that determines whether or not the CEL comes on. The downstream O2 sensor seems to be doing it's job, and the Cat just is not up to specs. I live in NY and the car was overdue for inspection. I had 2 choices: fix it or dump the car. But since the car could not pass emissions, who would buy it in my state? Decided to replace the Cat. It was a bitter pill to swallow - $1675 was the cost, but it did solve the issue and no problems since. You probably paid $300+ for the Cat and its probably working just fine, but not at the designed threshold.

If your state doesn't have emissions, there are other options...
 

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I hear ya. I think if the code comes back on again, I'll just do the "spark plug no fouler trick" I live in a rural town and they do not require emissions testing / passing anyway.
 

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OK... For you in your state, that's the move. For me in my state, my hands were tied and I was effectively "forced" into the repair.
 
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