89 Celebrity engine light when laboring

1989 Chev Celebrity, 4 cyl. auto 250,000+ km.
Engine light will come on when the engine has to work hard as in climbing a long grade. It goes away soon after leveling out again. I've tried reading
the code which seem to be code 12 and then four flashes which isn't in my chart.
Suggestions anyone? Thanks
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The code 12 repeated three times is the index marker - you pay attention to the code flashed in between the repetitions of 12. Are you quite certain it is just four even flashes? Is there any unequal spacing (timing) between the four flashes? For example, could it be one flash, pause, three flashes? That would be an error code 13 which is an O2 sensor error.
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-Mike-
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