2000 pontiac Montana... first I fix the heater holes that burst while driving and still overheated so I ran a test to see if the intake gasket had a leak it didn't so replaced thermostat and water pump and vacuum out and it still overheat

2000 pontiac Montana computer test reads code 405

Consider replacing the radiator. Passages may be blocked by phosphate scale and/or corrosion. An IR thermometer will help you locate blockages. You may need to reverse flush the cooling system, including the heater.

2000 pontiac Montana computer test reads code 405


If that DTC was a P0405, it's related to the EGR output control and likely has nothing to do with the issue of overheating.
Don't overlook the importance of the radiator cap itself. If it won't hold the specified pressure, the system will boil at a much lower temperature.