Used an O2 spacer. P0420

2005 BUICK LA CROSSE
207,000 MILES • 3.8L • V6 • 2WD • AUTOMATIC
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DC70
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Car has been well maintained with records. I got a P0420 code. Ran a couple treatments with Cataclean. P0420. Used an O2 spacer. P0420. Replaced O2. P0420. Replaced Catalytic converter. After ~120 miles, P0420. I'm Stumped!
Oct 18, 2025 at 11:52 AM
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STEVE W.
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OEM replacement converter? Federal car or Cali? Any exhaust leaks? Do you have a scan tool that can read the live data from the sensors? All stock, no cold air intake or other modifications?
Oct 18, 2025 at 2:01 PM
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KRAKENAUTO
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Was the vehicle doing anything out of the ordinary when you got this code previously, before you replaced parts?

Have you checked for exhaust leaks?

Also an easy way to test the system, is to pull the upstream o2 sensor out, install a back pressure tester, start the engine and see what the back pressure tester reads? This is a quick way to tell us if you have a bad catalytic converter or not and how to proceed.
Oct 18, 2025 at 9:49 PM
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