Not my usual topic, but I had the unhappy opportunity to review the law, and so I am sharing the benefit of my knowledge:
New vehicles: Generally the manufacturer’s written warranty, including exclusions, will determine whether or not the dealer is responsible for repairs on new vehicles. There is an exception, the so-called “federal emissions warranty” which is properly known as the Clean Air Act section 207 and codified at 42 USC 7541: http://codes.lp.findlaw.com/uscode/42/85/II/A/7541.
Amongst other warranties, 42 USC 7541 states that the manufacturer must warranty catalytic converter, electronic emissions control unit, and onboard emissions diagnostic device. These components are warrantied for a period of 8 years or 80,000 miles.
GM in particular seems to be having problems with catalytic converters. As long as you have properly maintained your vehicle (keep records), then GM is required to replace it. Take your vehicle to an authorized dealer and demand repair.
Used vehicles: Used vehicle service warranties are established by Alaska law. See Alaska Department of Law Consumer Protection Unit’s website: http://www.law.alaska.gov/department/civil/consumer/4545130.html.
Amongst other duties, a shop must provide a written repair order, a written estsimate, may not exceed the charges in the estimate without the customer's agreement, may not charge excessively, must return parts if requested by the customer when the repair order is place, and provide a written invoice when the repairs are completed.
Moreover, a shop may not refuse to release a vehicle if it is charging more than estimated without consent or charging excessivley.
If you are having a problem with a dearlership or a shop, you can file a complaint with the Consumer Protection Unit: http://www.law.alaska.gov/department/civil/consumer/cp_complaint.html.