You have a few options. I will list them for you so that you can decide on whether you want to pursue one, some, or all of them.
1. If they failed to perform the job and you have evidence that they failed to adequately provide the service, you can contact the original mechanic, provide proof, and demand free repairs to make the vehicle compliant with promises made.
2. Instead, you can also contact your new mechanic, have him make repairs, and then turn around and sue the first mechanic for fraud and for whatever overage costs you had when you were forced to make repairs. Typically you do owe a bit of a duty to permit the original party to fix the issue, but if you no longer trust them, you can go to someone else.
3. Likewise you can either threaten them (if they refuse to make repairs), or simply contact the Massachusetts Attorney General's office, speak with their consumer protection division, and file a grievance
directly over fraud and deceptive trade practices.
4. Finally, you can just turn around and sue for damages and costs.