A couple notes:
There *is* a known issue with the CAT getting plugged up, we experienced this on my wife'* 2001 Grand Prix GT, same L36 Series II as my Bonneville SE. The symptoms are not always "flows great" though, to the contrary, at highway speeds if I mashed the petal to the floor, the thing just choked, sputtered, and threw a Check Engine Light (CEL). I was mystified at this, and took it to my dealer who properly diagnosed it.

Something you should know: Catalytic converters are covered by a federal law that requires the manufacturer of the car (GM in our case) to warranty the thing for something like 8 or 10 years, and some large number of miles like maybe 100,000. Regardless of if you're the original owner of the car or not. (It'* different for me, I'm in California, and it'* slightly longer here than some other states, if all the info I received was correct.) My Pontiac dealership replaced it with a new one and sent the bill to General Motors. You might be out of that warranty, but still, just add it to your information pile.

Secondly, even if their CAT story were true, that it caused overheating of the EGR, causing your upper intake to melt and drink coolant, you STILL need an upper intake. DUH.

Good luck!

Yeah :p read closely, I said they want replace the entire UIM and LIM + both heads. The guy is really driving me nuts becuase he said it passed a compression test on all cylinders but he wants to pull the heads. Now, I am slowly realizing that he may be doing this to help me out, i.e. he pulls one of the heads and finds icky burnt coolant deposits in cylinders 4 & 6 and realizes that this is the crap that probably plugged the CAT. Whichever way I'm calling him in a 1/2 hour.

Update: I spoke with the service rep this morning, and thanks to my uncompromising views and stern but respectful tone I have had with them last night and this morning he has changed his mind, now believing that my UIM had been passing so much coolant that not all was combusted, running down the exhaust as a liquid and became a burning mess when it hit the ceramic in the CAT. I authorized removing the heads because due to a small external coolant leak on one of the heads, the tech is worried that I may have a bad head gasket or worse, I overheated it enough times to warp at least one.

He explained to me that he understands what the inspector is after, and he and the tech will be speaking little of the CAT, and if so that it was subsequently plugged and not the other way around, itè* not a covered item anyway. I told him that if everyhting goes my way, I will be supplying them with an aftermarket high-flow CAT. This will begin my exhaust mods.

Ah, okay, so they're going to cover your upper and lower intakes, well then if they want to do the heads, then fine. (I was afraid they were wanting to do your head gaskets but not the intakes!) But if they want to do your CAT, since your car is a 99, that should be covered by General Motors if you have less than 80,000 miles on it and are in the USA:

• Federal Emissions Warranty – Under the Federal Emissions Warranty, all emissions related parts are
covered for 2 years or 24,000 miles*, with the catalytic converter and engine control unit (the computer)
covered for 8 years or 80,000 miles*. These Federal Emissions Warranty provisions cover light-duty
vehicles up to 8500 pounds Gross Vehicle Weight Rating (GVWR).
•
California Emissions Warranty – Under the California Emissions Warranty, vehicles certified for sale
in California, and those states like Vermont that have adopted California Emissions Standards by law,
have additional coverage beyond that required under Federal law. Under the California Emissions
Warranty, all emissions related parts are covered for 3 years or 50,000 miles*, and a vehicle-specific list
of more expensive emissions related parts is covered for 7 years or 70,000 miles*. In addition, the 8
year or 80,000 mile* coverage for the catalytic converter and engine control unit required by Federal law
also applies.

Fantastic info, thanks a ton, everyone! I'll have to check locally, because unfortunately, my car finally had to be brought while I am here in Soviet Canuckistan :p

Yeah, I do just want to state how amazed I was when I found out about the Federally mandated coverage of the cat by the vehicle manufacturer.

When my wife'* Grand Prix was sputtering, gasping, choking, and coughing; I figured I was in for a big repair bill, either some expensive sensors, or a computer, or a major engine repair; having bought the car with only a 90 day warranty now long out of warranty. My Pontiac dealership called me up and said it was all ready to pick up, they had repaced the catalytic converter which had plugged, and there was a factory service bulletin out on it.

Figuring a dealership would have *the* highest price for a catalytic converter anywhere, I quietly asked how much this was going to set me back, and then the mechanic said "Catalytic converters are covered by the manufacturer under Federal law, so we billed General Motors, this will cost you nothing." I *really* wasn't expecting that!

Also, it'* good to know that there is some truth to the catalytic converters on our cars having a potential for getting plugged. I actually read the service bulletin somewhere, and it stated that the catalyst honeycomb material in the cat can break down and pieces clump together, clogging the cat, calling for replacement with a new cat. Knowing this lets you keep an eye out for any potential problems. While I personally doubt that it had anything to do with the EGR and the upper intake failure, there'* certainly nothing wrong with giving it attention and getting it taken care of *especially* if it is not at your expense!!

I'm really glad they know to do the upper intake and both the upper and lower intake gaskets. Sounds like you're getting the proper attention to the problem!

It'* all in how you look at it.... In the '92-'99 maintenance section there are @ 10,000 threads and @ 100,000 replies. If you don't find a cat "causing" the uppers failure, you can darn well bet it just ain't so. The NSA doesn't have databases this good.

Glad it worked out.

Thats why we call them the Stealership, they are allways trying to rip people off. There are some good ones though if you have a long standing relationship with them.

I am at my wits end with this warranty company. Backtracking, they are now repealing their claim that the CAT was the culprit, its now that because the EGR stovepipe is to blame which not covered by my warranty, NOTHING is covered. I stated my case that everything was functioning as designed, and it is a design flaw that caused this problem. I have gone 3 tiers up in their management and they're not backing down, simply because they have me by the balls at "the heat from the EGR system caused the failure, and parts from the EGR system are not covered, nor are any parts damaged subsequently". Now am stuck with the car in pieces with hours of labor involved. All not covered. I wish I knew more about this system or maybe who I could contact at GM to help me with this.

You need an Attorney.