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It is possible. I got a code 51 once that was caused by a MAP sensor whose output shifted (instead of outright failing). The MAP caused the engine to run rich, which made the O2 sensor go to one end of the scale and trip the code 51.
 

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The MAP sensor was relocated in 1987 to prevent moisture intrusion, so that problem is long gone. But they can and do drift in output voltage, usually to the rich side.
 

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With a code 21 and 51, I'd change the O2 sensor.
 
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