96 skylark i'm getting code P0420

[liquidflame1]

Hello all, I'm getting code P0420 catalyst efficiency below threshold (bank1)
is this an o2 sensor code???

[BNaylor]

To generate the P0420 DTC the 02 sensor is most likely working. On odb-ii cars the rear, post CAT convertor 02 sensor. It means your CAT has less than desireable oxygen storage capability. In most cases it is a bad CAT convertor.

[Blue Bowtie]

That's probably true on Buicks, but on Bonneville versions of the W-Body, I've had several P0420 DTCs on cars with weak rear O² sensors. Many years had a heated rear and non-heated front sensor. I'm not sure if the '96 was that way or not, but '98 Bonnies were like that.

[BNaylor]

Originally Posted by Blue Bowtie

Many years had a heated rear and non-heated front sensor. I'm not sure if the '96 was that way or not, but '98 Bonnies were like that.

On this particular Buick model regardless of whether he has the 3.1L V6 or 2.4L L4 both of the 02 sensors are heated.