3800 V6 computer codes P0420.
3 1/2 months ago mass air flow sensor changed, 3 months ago new catalytic converter.

Three and1/2 months ago I changed the mass air flow sensor and then had a new catalytic converter installed 2 weeks later. Car runs fine but now I have the check engine light coming on several months later.

Auto zone ran the handheld test and received the P0420 code for catalyst system efficiency below threshold(bank 1) Could it possibly be the O2 sensor?

It is possible that an
02 sensor is causing this problem. But since the replacement converter lasted awhile before the code came back, it may very well be that the catalytic converter is bad again.

This can be caused by a few different things. From engine running a little rich, to misfires (although you said the car is running fine), to poor / contaminated fuel quality, or just a cheap - poor quality converter. The $89 converter at the local exhaust shop have given us many problems over the years trying to correct repeat P0420 codes.

1999 Buick Lesbre 3.8 v6. Alternator and battery are about a year and a few months old.

My car died on me yesterday, i get power but not over 13 0r 14 on the volt meter and all lights interior and out are shining bright but the engine wont make any noise at all and no warning lights are coming on.

It could be a bad starter. If you've got decent
battery voltage and you're sure the battery is ok, you will need to check and see if you've got power at the starter solenoid wire (purple wire) when you are holding the key in the crank position.

If you have power there, then you know the starer is bad. If you do not have any power at the purple wire while cranking, then you have another issue. It could be anything from a faulty ignition cylinder or security problem, bad neutral safety switch or something more involved like a wiring issue. Check all fuses to make sure they are good, as well the starter relay.
1999 Buick Lesabre Engine Code P0420

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