Carfax and Autocheck BOTH only report information that is reported to them from their collection points. Nothing more. Therefore you can have garbage in and they will report garbage out as far as the information reported. Having said that it does NOT mean that these services are bad by any means it just means they are TOOLS/SERVICES to use to make your best judgement when purchasing a used vehicle.
Example: You go to have your car inspected for annual emissions or safety inspection and the inspector asks the you the owner "How Many Miles on the car" instead of looking for themselves. The owner says 65,000 miles without looking at the odometer yet the car has 95,000 miles showing and last years mileage report during inspection was correct showing 80,000 miles. Maybe the owner did look at the odometer however they did not have their glass's on and they reported the wrong mileage cause their blind! Now you have an odometer discrepancy! Same for the mechanic or anyone else who notes mileage, title information, accident information or any other information that is picked up and reported by these 2 reporting services.
Accidents may or not be recorded depending upon if a police report or insurance claim was filed. What about Hit and Run damage? That may or not be reported depending upon whether the owner has insurance coverage for that type of damage. The vehicle may have been totaled in an accident. Although the accident is reported by these services the vehicle was over 10 model years of age and the vehicle owner purchased their wrecked vehicle back directly from the insurance company.
In some states, Tennessee is one, where the insurance company does not have to put the vehicle thru the salvage process and therefore issue a salvage title to that vehicle if the vehicle is over 10 years old, there is no lien on the vehicle and the legal insured owner elects to purchase back their wrecked vehicle from the insurance company during the claim settlement process. Now you have a KILLED vehicle to rebuild with a good title and no salvage history only an accident report on Carfax or Autocheck. This is certainly the exception HOWEVER IT DOES HAPPEN!
Again these services are TOOLS to help you make your best judgement when purchasing a used vehicle nothing more. Look at the trend that you might see in the information reported with these services and see how it matches with what you can actually see on the vehicle in which you have an interest. Most mileage discrepancies on these reports are mistakes in odometer mileage reporting not outright odometer fraud.
Always remember whenever you have problems with any new or used motor vehicle dealer you can ALWAYS contact the Motor Vehicle Commission in your state and they will be glad to help you. Dealers do pay attention when the Motor Vehicle Commission comes calling for a problem/complaint filed by a customer. The Motor Vehicle Commission can fine them and pull their license.
Questions about a dealer? Call your state Motor Vehicle Commission.