I have since figured out that 2-3-1 is defined as:
LAMBDA ADJUSTMENT; FOR 1994 AND LATER, HEATED OXYGEN SENSOR (MIXTURE TOO LEAN UNDER PART THROTTLE)/ LONG TERM FUEL MIXTURE TOO RICH IN PART LOAD STAGE.

After all that, I am going to assume that one or more of my o2 sensors are bad.
How do I know which one to replace?

I could be misinformed or remembering this wrong, but I believe the O2 sensor just before the cat. provides metering data, while the one after the cat is used as a reference against the front one, ensuring resonable validity in data presented. My font sensor was showing a slow response time referenced to the rear, enough so as to trigger a fault code. After a while, these sensors will degrade and require replacement as a poster in this thread noted as 150k miles. I don't think OBD will present a code if both sensors degrade within allowences of eachother, thus the mileage recommendation.

I am not sure how many miles you have on this engine, but I would check all of the vacuum elbows and lines for leaks. Especially the one that goes around the engine on the passenger side. A vacuum leak causes the system to run rich and the O2 sensors object.
If you have over 150K miles, I would replace both sensors with original Bosh.

Klaus
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(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

Klaus;
I have 147,700 miles on the engine with original O2 sensors.
I think I will do what you say and replace both. That way I eliminate
any guessing game in the future as to which one is bad.
Thanks! Any ideas for a good price on these O2 sensors?

Advice? Read Rule302's answer and then check for vacuum leaks before dropping $300+.

Klaus
--
(V♂LV♂s 1975 164, 1995 854T, 1998 V70R)

The rear O2 sensor is there to provide catalyst monitoring. The front sensor will flucuate up and down as the oxygen content in the exhaust gas goes up and down. As the exhaust goes through the cat the left over O2 is used up in the process and the amount that comes out the other end is pretty consistent so that the rear sensor will show little to no flucuations. If the rear sensor starts to bounce up and down like the front one then it indicates that the cat is falling off and it will post a catalyst efficiency code. Hope this answers your question.

Mark