Hi MT, do you have the original catalytic converter on your Monte?

They are known for going back & throwing codes ?

I would call your local dealer & check if they will test it.

It should be covered under warranty.

Below is the

Federal Warranty Laws

1.The Magnuson-Moss Warranty Act (15 U.S.C. 2302(C))
This federal law regulates warranties for the protection of consumers. The essence of the law concerning aftermarket auto parts is that a vehicle manufacturer may not condition a written or implied warranty on the consumers using parts or services which are identified by brand, trade, or corporate name (such as the vehicle maker's brand) unless the parts or service are provided free of charge. The law means that the use of an aftermarket part alone is not cause for denying the warranty. However, the law's protection does not extend to aftermarket parts in situations where such parts actually caused the damage being claimed under the warranty. Further, consumers are advised to be aware of any specific terms or conditions stated in the warranty which may result in its being voided. The law states in relevant part:
No warrantor of a consumer product may condition his written or implied warranty of such product on the consumers using, in connection with such product, any article or service (other than article or service provided without charge under the terms of the warranty) which is identified by brand, trade or corporate name... (15 U.S.C. 2302(C)).
2. Clean Air Act Warranty Provisions (42 U.S.C. S 7541 (C) (3) (B))[/b]
The federal Clean Air Act requires vehicle makers to provide two emissions-related warranties a production warranty and a performance warranty. The production warrantyrequires the vehicle maker to warrant that the vehicle is designed, built and equipped so that it conforms with emissions requirements at the time of sale. The performance warranty[/b] requires the vehicle maker to warrant that the vehicle will comply with applicable emissions requirements as tested under state vehicle emissions inspection programs for the warranty periods specified in the law (for model year 1995 and later vehicles, the warranty is 2 years/24,000 miles for all emissions-related parts and 8 years/80,000 miles for the catalytic converter, electronic emissions control unit and on-board diagnostic device). The performance warranty is conditioned on the vehicle being properly maintained and operated.
Like the Magnuson-Moss Act, vehicle manufacturers may not refuse warranty repairs under the Clean Air Acts performance and defect warranties merely because aftermarket parts have been installed on the vehicle. The only circumstance under which the vehicle manufacturer can void the emissions warranties is if an aftermarket part is responsible for (causes) the warranty claim.

[:-]