Once again I'm going to link to Volvo's service manual on the LH-Jetronic 2.4 fuel system TP31361/1.
It holds all the fault tracing procedures for this fuel system and may be helpful in finding the problem.

If I were presented with this fault, what I would do is warm up the engine and measure the voltage on pin 24 of the ECU connector while connected to the ECU. You can do this if you take off the connector's sleeve (which is the recommended procedure anyway) and reinsert it into the ECU conn.
A voltage of approx. 0.5 V is to be expected with the ignition on and a voltage that keeps swinging between approx. 0.1 and 0.9 V with the engine idling at normal operating temperature.
Alternatively, you could measure it at the firewall connector, but I prefer to do it at the ECU to include all the wiring between it and the O2S.
Good ground connections are important. On a 200-series, the ECU grounds on the inlet manifold, it has no ground rails closer by like on the 700-series, as I understand it from Dave's post.

If values were false, I'd first check if the heater part of the O2 sensor was getting 12 V through the yellow-red wire before condemning it. The heater part can be checked for by resistance measurement. 3 Ohms at 68F, 13 Ohms at 660F (idle), see section E23 of the manual.

The biggest challenge here is the intermittent nature. That type of fault may take a lot of time to track down. Carry a meter and some tools with you. I sometimes hook up a meter permanently to see what the values do while driving. The min/max-hold function (if equipped) on the meter can be useful to keep your focus on the traffic instead of on the meter.

Also very useful is this website.

Is 2-1-2 the only stored code, or are there any more if you repeat the read out directly after getting the first code? What about codes on the other socket (#6)?


Good luck! Please report back what you find.