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P420 code. Replaced CAT and O2 sensors. Still not fixed. Any ideas?


Full Member
Buick Ownership
Got a p420 code (catalytic system low efficiency). On a 2004. Only 3 things can cause this as far as I know: 02 sensors, Cat. Or in RARE situations, bad ETC sensor. The car has fits of rotten egg smell, rough running and misfires.

Live data voltage from 02’s while running were in range.
So I replaced Catalytic converter.
Problem persists.
Tested ETC sensor. It’s good.
Replaced both O2’s Incase my testing was faulty.
Problem still present.

I’m out of ideas.

Side note. I’ve replaced the Ignition Control Module 2 times in this car. Each time it presented with similar symptoms (except bad smell) and also triggered unrelated trouble codes. I would think this is the same thing but the sulfur smell has me stumped.


Full Member
Buick Ownership
1996 Roadmaster Limited Collectors Edition 58k - 1996 PAU


Full Member
Oakland Gardens, N.Y.
Buick Ownership
98 Riviera, 70 GS455 Stg1
Rotten egg smell is from running rich. You need to have the engine scanned and make sure the fuel trims are correct and to check the misfire history. A misfiring cylinder puts gasoline into the catalytic converter and will overheat it and ruin it.

Some replacement converters aren't as efficient as OEM so they set this code. The simple fix is an O2 sensor extender. I have used one for 6 or 7 years now. Like this,

Amazon.com: O2 Sensor Spacer Adapter Extender: Automotive

Read the reviews, people use this all the time for that engine code.

First make sure your engine is running correctly. Sounds like you might have other unresolved problems. I would clean the MAF and replace the air filter for starters.
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