The OBD-II code P0446 often triggers along with other related EVAP codes (such as P0441), and the specific combination of codes can give your mechanic a better idea of where the actual problem lies.
What the code indicates
More specifically, P0446 indicates a malfunction in the vent control circuit of the Evaporative Emission Control (EVAP) system. Generally, this points to a faulty vent valve, a bad control circuit for that valve, or a blockage in the vent valve. In any case, it clearly indicates to the mechanic that an inspection of the EVAP vent valve is necessary.
The job of the vent valve is to seal the vent so that the EVAP system can pressurize and check itself for leaks. Battery voltage is typically supplied to the vent valve when the key is turned on via a ground circuit in the Engine Control Module (ECM). The valve closes when activated, allowing pressure to build up in the EVAP canister. If the valve does not close or if a vacuum cannot be maintained during this test, code P0441 will trigger in the ECM.
How to fix code P0446
To resolve this issue in your Toyota, you will need to test the vent valve circuit from the ECM to the valve to ensure it closes and receives power when the key is turned on (the engine does not need to be running). Check the resistance on the control circuit against the specifications in your manual. If everything is in order, replace the vent valve and reset the ECM/PCM codes.
If other codes are also triggering, before replacing the vent valve, check them to see if the problem is not further upstream and causing the P0441 as a trigger.