It is not reliable at all, in fact avoid it and use other alternatives.  ABC’s Diane Sawyer exposed just exactly what Carfax is.  I will link you to the ABC story but if you don’t want to read both pages, I will get to the main part of the article that you need to know that Carfax is far from reliable and not to be trusted at all. Make sure you watch video below too. carfax sucks avoid carfax

It wasn’t just this one dealer. The ABC News investigation revealed dozens of used cars with clean Carfaxes — even though the cars had been in accidents, and others had frame damage, according to Autocheck. Like Carfax, Autocheck also includes a disclaimer about its comprehensiveness, but it does boast one important source of information that Carfax doesn’t have: Car auction data.

ABC News returned to the New Jersey dealership, armed with the police report saying the Mazda had been in a major accident.

The salesman defended himself by saying, “I don’t get under the hood of these cars or anything like that. I print out the Carfax. … If I print out a Carfax and it says that it’s green, that means the Carfax is 100 percent.”

http://abcnews.go.com/Business/trust-car-fox-hidden-camera-shows-clean-carfax/story?id=18731208&page=2