If the auto repair shop performed repairs on your vehicle that you did not authorize, then they engaged in fraud. If you received a part that had the ID number ground off, it raises the strong possibility that this is part of a larger criminal enterprise (colloquially referred to as a "chop shop" where stolen vehicles are dismantled and the parts are sold to disreputable auto repair or body shops to be placed on unsuspecting customer's cars).
You can contact local law enforcement to report your suspicions, call their non-emergency number and ask to speak to a detective or officer to report a possible crime.
Regarding compensation for the failed part, this is dealt with under "breach of contract" and "negligence" theories.
There is a series of escalating things you can do to deal with this situation.
- *First: start with the company's customer service and dispute the claim. Keep your complaint in writing. If you speak to someone by phone, follow up promptly with a "confirmation letter" (see my note below).
- *Second: (you can do this at the same time), if you paid by credit card (not debit card) you can open a dispute with your credit card carrier (follow the instructions with your credit card company). (Some banks do allow for charge disputes on your debit card - but not all, the laws are not the same and debit card purchases do not have the same protections, if you paid by debit card, contact your bank and see if they do have dispute resolution remedies).
- *Third: open a dispute with the BBB. The BBB offers consumer dispute resolution that is fast, free to consumers, and is usually effective, they have no enforcement authority, but all BBB disputes result in a public report regarding resolution so businesses do respond to them. You can open a BBB dispute here: bbb.org/consumer-complaints/file-a-complaint/get-started
- *Fourth: if you believe that the company is acting fraudulently (not just charging high rates), you can report them to the state Attorney General. The AG's office does not prosecute individual claims (so they will not get your money back for you), but they will investigate and potentially take administrative and/or criminal action against the company.
- *Finally: you can file a small claims action against them for breach of contract. Small claims actions take approximately 3-8 months to go to trial. There is no guarantee of success in these disputes, but filing a small claims action does open an opportunity to negotiate a resolution (in addition to the above opportunities and can lead to mediation - many courts offer mediation programs for their small claims docket).
Confirmation letters: Keep written records of all communications - so if you speak to someone by phone, promptly send a follow up "confirmation letter" summarizing your conversation, who you spoke to, when, and any agreements you reached. Keep copies of your outgoing correspondence, as well as anything that you receive.
I wish you the best of luck with this dispute, and hopefully a speedy resolution.