2-1-2 on OBD port 2 is signal missing/faulty from oxygen sensor. You've replaced the sensor once in an attempt to deal with it being a faulty signal, so perhaps think a little harder on the missing part.

I checked out the discussion in your O2 sensor problems thread from a year ago. Based on all that you've done so far and what you're now saying, the problem smells to me like an intermittent missing signal, possibly even seasonal in nature. I'm very much guessing you've got a poor electrical contact somewhere in the O2 sensor wiring circuits.

Art mentioned carefully inspecting fuse 4 which is for the O2 sensor heater circuit and fuel pump. That fuel pump circuit is one of the more notorious problem areas as it is high current draw, especially with aging fuel pumps.

I'll further suggest checking every connection in the O2 wiring starting at the firewall connector and working your way back to the ECU. Peel back the rubber connector boots to make sure the pins are solidly pushed in from both sides -also pins at the large main connector blocks.

Anytime I see mention of intermittent seasonal behaviour with 240s, I immediately think of possible chassis ground problems. With that in mind, I'd be very, very suspicious of the black wire having a solid chassis ground connection. You can start by checking with a meter, but I'd want to do a proper inspection. I don't recall where the O2 sensor black wire attaches in the LH 2.4 240s, but for a 740 it goes to the ground rails behind the ECU. Make sure all chassis ground connections in such areas are clean and solid - wiggle and remake connections as needed.

Is there any chance someone has ever had the ECU connector off for diagnostic testing or replacement? I'd want to pull the connector for inspection and reseat it, including removing the connector shell to make sure no wiring is damaged. The O2 sensor (pin 24) is one circuit that technicians sometimes want to measure there. If people aren't careful with probes, the little contact fingers can get bent and make for a poor connection. Dirt, like dust and hair getting caught in an opened connector can also be a problem. BTW, as much as the original '89 LH 2.4 ECUs are well known as troublesome, I've not of heard of the onboard O2 sensor circuit being a problem.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now