I have 3 of these fixtures.
I have found all my issues to be tied to my bulbs. It is easy to purchase bulbs that seem right, but wont work.
Check to make sure you have actual honest to goodness T5 HO bulbs in it. I have twice gotten the wrong bulbs for mine, it is super easy mistake to make.
From what I understand of electronics, I think the 2sec on 2sec off is a trouble code for the fixture.
If you have a friendly petstore near you, you could take the whole fixture in, and ask if they would try working T5 HO bulbs in at at the shop, and if that is the issue, you can just leave them in and buy them.
That is what I would try, avoid replacing the fixture if you can, they are not inexpensive.