I was about to just upvote @nihilist_frost but I have a few quibbles with his answer.
First: You can be "certified in" a subject. Like "I am certified in diesel engine repair." You can be "certified on" a particular machine or tool. "I am certified on turret lathes." You can be "certified as" a profession. "I am certified as a hairdresser."
Second: If you say, "I am ABC certified", "ABC" could be the organization that gave you the certification, or it could be the subject matter. Like, "I am Microsoft-certified", or "I am Java certified."