ODBII port is under the dash just to the upper right of the emergency brake.
2000 Chevy Silverado LT Ext Cab 4 dr 5.3L 4x4 Z71.
Honestly if you don't know where the drain plug is, don't mess with the vehicle then!
The plug wires could be connected wrong. If that's not it, you'll need to check the diagnostic code.
Locate the plug on the bottom of the radiator to drain it. Put some radiator cleaner in and drain it again.
A 2009 Chevrolet Silverado with the 5.3 engine requires a .040" spark plug gap.
Just below the steering wheel tube and off to right there should be a connecter for diagnostic plug.
2006 Chevrolet Truck Silverado 1500 2WD 5.3L SFI HO 8cylThe Spark Plug Gap.040 (In thousandths of an inch)
on the oil pan
firing order is 18436572
.045
Straight from the service manuals:2000 Silverado 2500 6.0 spark plug gap should be:1.524 mm (0.060 in).
bottom passengers side of radiator
It is on the bottom of the oil pan, and the oil pan is on the bottom of the engine