I have been talking about the self diagnostic stuff here lately but don't get the impression I'm an expert. I'm just reporting what Bentley says on the subject.
But, what code 2-1-2 is reporting is a fault in the O2 sensor signal or the absence of any signal at all. Which does not necessarily mean the sensor is bad. In your place I would have checked the O2 sensor itself before buying another. If it tested normal I would have checked the wiring harness connector at the ECU for continuity between the test point for terminal 24 and ground. With the oxy sensor connected there should be no resistance. With the sensor disconnected and the green wire grounded there should be infinite resistance (be sure to protect the connector on the black wire to the sensor when you disconnect it, don't let it touch metal). If those conditions are not met I don't know what to tell you other than something must be wrong in the wiring. If those conditions are met then I would think the problem is in the ECU.
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'80 DL 2 door, '89 DL Wagon