What do these codes mean on a plymouth neon: 1684, 0051.?

Then engine light in my car started coming in the other day, and I can't figure out why. I did figure out how to get the code of what is wrong with it, but I don't know what the codes mean. I drive a 2000 Plymouth Neon. The codes are 1684 and 0551. Can someone please tell me what they stand for. Thanks.