Emissions Code

The subject car is a 1996 Pontiac Grand Prix with 115,000 miles and a v-6, SFI 3100 engine. I am told there are 2 Upstream O-2 sensors, and 1 Downstream sensor. I have replaced 1 Upstream sensor, the one that is located in the center rear of the engine closest to front seat area, but cannot locate the second Upstream sensor.
The Check Engine light remains on and the Codes are now P0300, Random/Multiple Cylinder Misfire, and P0420, Catalyst Efficiency Below Threshold-Bank 1. I would replace the other Upstream sensor if I could find it.
Questions: A.- is it possible there is only one Upstrem O-2 sensor with this model engine? If there is a second sensor, where is it ?
and, B.- given the above information, what does one think is the issue with the 0420 Code?
Much in need of assistance as the tags have expired and I need to pass an Emission Test. Thank you in advance for any help on this matter.

There are only 2 O2 sensors.One is located in the rear exhaust manifold and the other is downstream,which is located after the cat converter.If you replaced the O2 sensor with a Bosch that could be your problem and should stick with an AC Delco AFS75.

As for the Missfires id start with some plugs and wires.

~Larry~

There are only 2 O2 sensors.One is located in the rear exhaust manifold and the other is downstream,which is located after the cat converter.If you replaced the O2 sensor with a Bosch that could be your problem and should stick with an AC Delco AFS75.

As for the Missfires id start with some plugs and wires.

~Larry~

I agree with Larry, the Vin M 3.1 V6 has two 02 sensors. The one after the CAT is AC Delco part number AFS97.

I've had odb-ii DTC error P0420 twice. Once in a '99 Regal and one time in my '97 GTP. Both times replacement of the CAT convertor resolved the issue.

Welcome back Larry. Drop KustomKid54 (Jim) a line. He is not doing too well.

There are only 2 O2 sensors.One is located in the rear exhaust manifold and the other is downstream,which is located after the cat converter.If you replaced the O2 sensor with a Bosch that could be your problem and should stick with an AC Delco AFS75.

As for the Missfires id start with some plugs and wires.

~Larry~

Thank you for your assistance, I'll stop looking for a second sensor. I did replce the Upstream sensor with a Bosch product, wonder why it would/could make a difference? Meanwhile I'll go ahead and replace the Bosch sensor.

The plugs and wires have already been replaced. The car has the most problem when, for example, stopped at a light. The car almost wants to quit running. Although this could be a result of a too lean mixture, I'm concerned this may be a sign of a problem a lot worse than an O-2 sensor.

How does one approach the problem after replacing the plugs and wires? What's the next thing to look at? Thanks, Bill

I agree with Larry, the Vin M 3.1 V6 has two 02 sensors. The one after the CAT is AC Delco part number AFS97.

I've had odb-ii DTC error P0420 twice. Once in a '99 Regal and one time in my '97 GTP. Both times replacement of the CAT convertor resolved the issue.

Welcome back Larry. Drop KustomKid54 (Jim) a line. He is not doing too well.

Thanks, I'm going to replace the new Bosch O-2 sensor with a Delco product. I expect the converter will have to be replaced, but I'm looking to correct the more minor problems first. Bill