The P0446 is pointing to a fault in the EVAP Cansiter Vent Valve. Inspect the vent valve vacuum lines for any disconnections, holes or kinks. Check the electrical connector for any loose or broken wires. If the visual inspection checks ok, suspect a faulty Vent Valve.
The P0420 is in a totally different system, and more than likely, unrelated. The P0420 is most commonly the sign of a failing Catalytic Converter. The oxygen sensors measure the amount of pollutant going into, then out of the converter. When they report to the PCM the values, and these values are below what the PCM is pre-programmed to see, it's determined the convter is losing it's ability to perform it's job correctly, and will need to be replaced.
If thats the case- then you now know why you have a P0446 code! You'll need to obtain a new one from a GM Dealer. "Emissions" related parts are not offered in after market parts stores.... yet...
And there are no broken, disconnected or holed vacuum lines?!
DESCRIPTION A restricted or blocked Evaporative Emission (EVAP) vent path is detected by the Powertrain Control Module (PCM) monitoring the Fuel Tank Pressure (FTP) during normal operation. The PCM commands the Evaporative Emission (EVAP) vent valve OFF (open), and the EVAP canister purge valve ON (open). The fuel tank pressure should remain low as vacuum is drawn on the EVAP system. If the vent path is blocked or restricted, the vacuum level will increase and DTC P0446 will set.
DIAGNOSTIC AIDS The EVAP system tests run when the engine is first started and meets the Conditions for Running the DTC. An intermittent condition could be caused by a damaged EVAP vent housing, a temporary blockage at the EVAP vent valve inlet or a pinched vent hose. A blockage in the vent system will also cause a poor fuel fill problem.
Correct. It's a fairly straight forward code. Most commonly there is a pinched or kinked vacuum line. Also very common is the vent valve itself cracks. Definitely re-check those vacuum lines.