2001 Century 3.1 trouble code P0420 cat conv low threshhold reading problem


New member
Buick Ownership
2001 2002 century
I have a problem with my daughters 2001 century 3.1 , I had an emission code come up about a year ago P0420 . I changed the cat converter with an aftermarket one and it solved the problem for at least 8 months. Now the code came back. The O2 Sensors are both reading real time data like the should, I put a new one in post converter when I changed the cat last year. Im thinking I just might a cheap aftermarket converter that went bad , It cost me around $120.00 on E-bay. The prices are low , cheap and expensive , Is there a main difference on which one you choose. Also has anyone had any luck adding laquer thinner to the fuel and driving on the highway for a tank full and had any luck on clearing up the converter? Any help will be much appreciated. Thank you, Old Car Guy


Well-known member
S.W. Ontario Canada
Buick Ownership
2018 Regal Sportback - 2010 Yamaha R6 (track bike) Past cars: 92 LeSabre, 98 Regal, 02 GrandPrixGT
Don't do the lacquer thinner thing. If anything, just get the stuff they sell in the store to clean the catalytic converter. Same idea as with the Lacquer thinner... Add to tank right before filling up, then drive whatever the instructions say.

Aftermarket Catalytic converter I've had superb luck with is Magnaflow... But a muffler shop will have to weld it in. Obviously get the Magnoflow converter with the right diameter inlet/outlet.

BTW,.. what brand rear 02 did you install last time ???
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