[Bob Lincoln]

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.

 

[B10alia]

If you're looking at the moisture issues section for the MAP sensors on the troubleshooting page, that information is out of date. By '94, the location of the sensor had changed, and they were no longer susceptible to the moisture accumulation issue. You could be looking at a failed sensor, or a cracked vac line. There's a hard plastic section in the MAP line that can crack, and rubber deteriorates over time.

 

[Bob Lincoln]

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.

 

[B10alia]

To the OP:
Have you solved your problem?

 

[VillageIdiot]

Not yet. It still throws a code 21 when I start it up, but if I drive it for a minute or so then shut it off and turn it back on, the Check Engine light goes out

 

[B10alia]

Code 21 is for an oxygen sensor. If it has never been changed, it may well need to be. The MAP sensor being out of range could potentially cause the MAP to seem to the computer to be faulty. There's always a bit of confusion about oxygen sensors and these codes; I defer to Bob Lincoln, since he wrote the article I'm basing this off of.

 

[Bob Lincoln]

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