OBD II Trouble Code P0430 relates to your truck's Bank 1 (Side of engine where number 1 cylinder is located) Catalytic Converter (CAT). Your 2001 GMC Sierra's powertrain control module (PCM) monitors Catalytic Converter efficiency using heated oxygen sensor (HO2S) bank 1 sensor 2, which is located in the exhaust stream immediately AFTER the CAT. The HO2S bank 1 sensor 2 produces an output signal which indicates the oxygen storage capacity of the catalyst. This determines the Catalytic Converter's ability to effectively reduce harmful exhaust emissions (HC, CO, and NOx). If the CAT is functioning properly, the bank 1 of the HO2S 2 voltage will be far less than the voltage produced by bank 1 of the HO2S 1 oxygen sensor. This indicates that the CAT oxygen storage capacity is below an acceptable threshold; and OBD II P0430 will set.
This trouble code could be set due to a defective rear oxygen sensor or defective Catalytic Converter. Before suspecting the CAT we'd recommend replacing the rear oxygen sensor (bank 1), or perhaps at switching rear oxygen sensor bank 1 with rear oxygen sensor bank 2 as a test. If the check engine light fails to clear over the course of 3 drive cycles (driving conditions required to run a trouble code test for P0430), then you may suspect a defective Catalytic Converter.