How do I scan for codes? on 1999 Cadillac DeVille

Changed Converter last year and told needs to be replaced again. Not sure about that, cleaned gas cap and check engine light went off for about 100 miles and then came back on. Please help quickly before my husband runs out and gets another converter. It can't possible need to a new one every year.

Asked by for the 1999 Cadillac DeVille
Take it to auto zone and they will scan it for free, then get the cades and post them here so we can help.