I GOT A 2000 SILVERADO 1500 OBD CODE IS 421 THE PROBLEM IS I DON'T KNOW LOCATION OR HOW TO FIX THE PROBLEM

Sounds like an O2 sensor, especially if your not experienceing major problems.

Example obd code P0101
1st digit: B-Body, C-Chassis, P-Powertrain, U-Network
2nd digit: 0-SAE (generic), 1 mfg (Manufacturer Specific)
3rd digit:
* 1 Air & Fuel Metering
* 2 Air & Fuel Metering Injector Circuit
* 3 Ignition System (Including Misfire)
* 4 Auxiliary Emissions Controls
* 5 Vehicle Speed Controls And Idle Control System
* 6 Computer Output Circuit
* 7 Transmission
* 8 Transmission
4th & 5th digit - two place fault code 0-99