If the CEL is blinking, it's indicating that there's a misfire. Get the codes read. In your case I'd be looking at the cat.
You were right. It was the cat. Interestingly enough, the dealer told me that it didn't fall under warranty coverage (the 8 year portion expired last September). And that it would be $750 to repair. After some more digging, I came across some GM documentation that extended warranty coverage on all 2001/2002 3.4/3.8L Engines.
So that meant it got fixed for free!
Special Policy Adjustment - Catalytic Converter # 05551 - (Sep 14, 2005)
2001-02 CHEVROLET IMPALA, MONTE CARLO
2001-02 PONTIAC GRAND PRIX
2001-02 BUICK REGAL
EQUIPPED WITH 3.4L (RPO LA1 - VIN E) OR 3.8L (RPO L36 - VIN K) V6 ENGINE
Some customers of 2001-02 Chevrolet Impala, Monte Carlo; Pontiac Grand Prix; and Buick Regal model vehicles, equipped with a 3.4L (RPO LA1 - VIN E) or 3.8L (RPO L36 - VIN K) V6 engine, may experience a condition where the vehicle exhaust catalytic converter is replaced due to complaints of lack of power or illumination of the Malfunction Indicator Lamp (MIL). This may be due to the front endcone insulation from the catalytic converter breaking away and blocking the front of the first catalyst brick and preventing the free flow of exhaust gas through the converter.
Special Policy Adjustment
This special policy covers the condition described above for a period of 10 years or 120,000 miles (190,000 km), whichever occurs first, from the date the vehicle was originally placed in service, regardless of ownership. The repairs will be made at no charge to the customer.
For vehicles covered by Vehicle Service Contracts, all eligible claims with repair orders on or after September 15, 2005 are covered by this special policy and must be submitted using the labor operation codes provided with this bulletin. Claims with repair orders prior to September 15, 2005 must be submitted to the Service Contract provider.
2001 Chevrolet Impala 19100001 - 19378250
2001 Chevrolet Monte Carlo 19100001 - 19378233
2001 Pontiac Grand Prix 1F100004 - 1F275188
2001 Buick Regal 11100004 - 11338085
2002 Chevrolet Impala 29100001 - 29374833
2002 Chevrolet Monte Carlo 29100016 - 29374832
2002 Pontiac Grand Prix 2F100019 - 2F276927
2002 Buick Regal 21100001 - 21288762
Involved are all 2001-02 Chevrolet Impala, Monte Carlo; Pontiac Grand Prix; Buick Regal model vehicles, equipped with a 3.4L (RPO LA1 - VIN E) or 3.8L (RPO L36 - VIN K) V6 engine and built with the VIN breakpoints shown.
10343187 Converter, Catalytic (L36 engine)
10343185 Converter, Catalytic (LA1 engine)
10328740 Gasket, Catalytic Converter (Converter to I-Pipe, both)
15286606 Gasket, Exh Manif Pipe (L36)
24505057 Gasket, Exh Manif Pip (LA1)
Parts required to complete this special policy are to be obtained from General Motors Service Parts Operations (GMSPO).
General Motors will notify customers of this special policy on their vehicles (see copy of typical customer letter included with this bulletin - actual divisional letter may vary slightly).