That's my trouble code. I don't know if it's being set because I am running one step colder plugs OR that it's just seen better days and needs replaced. I tried clearing the code with an OBD II scantool and it came back on after 100 miles. I just hope that it's the sensor alone and nothing else.

Don't know if the bonnevilles have them but have you checked your O2 sensor behind your cat? I get this code every once in awhile (but that's because I actually removed my cat haha)

P0420 DTC Code - Catalyst System Efficiency Below Threshold

Not a major issue other than emissions... just an annoying Check Engine Light. haha

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1997 Grand Prix GTP "Black Ice, 'cuz it's slick" - 17" Motegi Racing Rims, Hankook Optimo Performance Tires, 'Big 3' Wiring Upgrade, Weapon R Secret Weapon Intake w/ Ram Air, Larger SuperCharger lines, AutoLite 104's, U-Bend Delete, Resonator Delete, Cat Delete, Turbo Flowmaster Mufflers. 342bhp, 327 lb-ft torque, 9.091 boost psi @ 6,012 rpm.