Carfax doesn't investigate. Internet
I went to trade in my perfectly maintained 2005 Toyota Sienna and found out, when the first car dealer pulled the Carfax, that they had the van down as having been involved in an accident in Feb. of 2010 in which the airbag was deployed. The van had never been in any accident, of any kind.
I filled out the online form at carfax.com that allows you to inform them of an error on your report. The form asks you for "documentation" to prove they've made an error. Well, I have no documentation of having NOT been in an accident. Does anyone?
The form said I'd hear from them but, four days later, still nothing. Meanwhile, I've been all but called a liar by one salesman (who will obviously not be getting my business.)
The dealership where we've had the Sienna services has stepped in though when I told them of my problem and discovered the source of the "accident report." An airbag was found to be defective in the van around the time of the "accident" and was replaced by them.
Apparently Carfax gets its info from two sources: police reports and PARTS ORDERS. The airbag was ordered so Carfax assumed it had been deployed and there'd been an accident.
I'm still in shock that this company, who ONLY deals in information, doesn't even check out the info their getting! This may have cost us thousands of dollars and, from what I've read online, HAS cost others that and more. I will never trust them and will tell everyone I know about this. Carfax reports are utterly worthless.