EVAP system control circuit malfunction
The P0446 OBD-II code often triggers with other related EVAP codes (like P0441), and the specific combination of codes can give your mechanic a better idea of where the actual problem lies.
Specifically, P0446 indicates an Evaporative Emission Control (EVAP) system vent control circuit malfunction. Typically, this indicates a faulty vent valve, a bad control circuit for that valve, or a blockage in the vent valve. In all cases, it clearly shows the mechanic that a check of the EVAP vent valve is in order.
The vent valve's job is to seal the vent so that the EVAP system can pressurize and check itself for leaks. Battery voltage is usually supplied to the vent valve when the key is on through a ground circuit in the engine control module (ECM). The valve closes when activated, which allows pressure to build in the EVAP canister. If the valve fails to close or a vacuum cannot be maintained during this test, the P0441 code will trigger in the ECM.
To fix this problem in your Toyota, you'll need to test the vent valve circuit from the ECM to the valve to ensure that it's closing and receiving power when the key is on (the engine does not have to be running). Check resistance on the control circuit against your manual's specifications. 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 those to see that the problem isn't further up the line and causing the P0441 as a periphery trigger than than the primary problem.