97 dodge stratus check engine light

Hi my check engine light goes on and off. When I run the self-onboard diagnostic test I always get the codes 12, 21, and sometimes 31. I believe 31 is the emmisions code but the other two I don't know. Any help, ideas, or solutions. Thank you.

check out this thread (http://www.automotiveforums.com/vbulletin/showthread.php?t=355082), it should answer your questions.