It shows SOME facts.

I brought an OB2 reader off Amazon and it lit up the moment I plugged it into this used car I was looking at.

Buying a used car off a private seller is outright scary if your not a car person. At least at a car dealership I can tell how much I'm getting screwed

Carfax has effectively marketed itself as a single source of truth for vehicle history. However, Carfax reports may have omissions and errors, especially as vehicles age out of the manufacturer's warranty. It's best when used to inform and supplement other forms of research, such as a pre-purchase inspection or review of paper records.