2002 Buick Century Code P0420

DTC P0420: Catalyst System Low Efficiency Bank 1
Circuit/System Description
Important: A new converter with less than 100 miles on it may set DTC P0420 due to out-gassing of the internal matting. Operating the vehicle at highway speeds for approximately 1 hour may correct the condition.
A 3-way catalytic converter (TWC) controls emissions by converting hydrocarbons (HC) and carbon monoxide (CO) into carbon dioxide (CO2), and oxides of nitrogen (NOx) into nitrogen. The TWC also stores oxygen. When the listed idle conditions follow the listed driving conditions, the Catalyst System Efficiency test will run. The powertrain control module (PCM) monitors this oxygen storage capacity by comparing the pre and post-catalyst oxygen sensor signals while adding and subtracting fuel during specific idle conditions. The PCM compares the pre and post-catalyst-oxygen sensor signals to determine if the oxygen storage capacity of the catalyst is degraded.

https://www.youtube.com/watch?v=CflIvd5VOoA&t=293s

P0420 OBD-II Trouble Code: Catalyst System Efficiency Below Threshold (Bank 1) P0420 code means that either the upstream or downstream oxygen sensor in the catalytic converter is not working. In short your catalyst is worn out.

You need to put a scanner on the car and clear the codes and see if they come back. Normally you need to drive approx. 50 miles for the computer to cycle through all of the checks it makes.

Hey Frank how's it going ? Most usually A vacuum leak causes a P0171 lean code ! So I thing your cat is worn out ! P0420 How To Diagnose Bad Catalytic Converter EricTheCarGuy

code p 0420 refers to cat converter system bank 1---efficiency below threshold====causes ---cat converter---wiring---HO2S (heated oxygen sensor, the one in the exhaust manifold before the cat converter)
code p 0300 is random misfires
I would suggest changing the HO2S as it reports to the ECM on exhaust content so that the ECM can adjust the air/fuel ratio
so it is logical that if that unit is not reporting correctly, the cat converter may be having trouble burning off the gasses correctly
The oxygen sensor(O2S) reads the results of the cat converter operation and helps fine tune the ECM settings

Yes it is possible if the injector cleaner had any amount of lead in it. Most often it's the catalytic converter that is the problem, and causes the sensor to set the code.
I hope this helps.

Reset the ECM by taking out the negative battery terminal off your battery and leaving it aside for several minutes.

Are you talking radio code? If so you need to call Buick dealer and provide your VIN# to get code for theft deterrent radio, they're the only source.

Hi and welcome to FixYa!

Code p0420 means Catalyst System Efficiency Below Threshold (Bank 1). This will surely needs attention to resolve the problem.

Hope this helps and thank you for using FixYa! Have a good one!

here is a site that may help you with your po420 code http://www.aa1car.com/library/p0420_dtc.htm