96 Dodge Caravan Trouble Codes

I have a 96 Dodge Caravan 3.0 liter with 174,000 miles on it. It runs fine but shows a trouble code sequence of 12 21 55. I think the 12 means the battery is connected and the 55 is the end of sequence, but what does the 21 mean and what can I do to fix it? Thanks.

Its show an faulty oxygen sensor. Here is the link to the code.
http://www.allpar.com/fix/80s-codes.html