Should I get a computer diagnostic test for my car?

I have a 2004 Chevy Aveo and the check engine light comes on and then goes off at random moments. When it first came on, I went to the shop and they used the handheld machine, but didn't get any code. After that, it went off, but then came on again. They suggested the computer test at a cost of $89.99. I was looking into getting it done before because whenever I turn on my defroster, my car stalls and sometimes shuts down. Any advice given will be greatly appreciated. Thanks!