a quick google of the M-M act and I do see where you have the idea the GM needs to diagnose. I still dont think you are going to get far with it, but here is some info that may help you.
from: Magnuson-Moss Act
, not sure if this is out of date
"Overall, the laws governing warranties are very clear. The only time a new vehicle warranty can be voided is if an aftermarket part has been installed and it can be proven that it is responsible for an emission warranty claim. However, a vehicle manufacturer or dealership cannot void a warranty simply because an an aftermarket equipment has been installed on a vehicle.
If a dealership denies a warranty claim and you think the claim falls under the rules explained above concerning the clean air act (such as an emission part failure), obtain a written explanation of the dealers refusal. Then follow the steps outlined in the owners manual. However, if this fails, then phone your complaint in to the EPA at (202) 233-9040 or (202) 326-9100.
If a dealer denies a warranty claim involving an implied or expressed new car warranty and you would like help, you can contact the Federal Trade Commission (FTC). The FTC works for the consumer to prevent fraudulent, deceptive and unfair business practices in the marketplace and to provide information to help consumers spot, stop and avoid them. To file a complaint, you can call toll-free, 1-877-FTC-HELP (1-877-382-4357), or use the online complaint form. The FTC enters Internet, telemarketing, and other fraud-related complaints into Consumer Sentinel, a secure, online database available to hundreds of civil and criminal law enforcement agencies worldwide. "
So, looks like your next stap is to call the EPA