With Toyota/Lexus stability control light and Check Engine light often come on together (depending on the code stored) . A scan tool is required to diagnose the problem .A scan tool is attached to a 16 pin under the dash, the computer in your car stores in its memory what it sees as being the problem with your car. Any auto repair shop or Toyota dealer will have the scan tool needed to diagnose the problem. Fixing the Check Engine light problem will usually "fix" the VSC code as well.
P0446 signifies a fault in the charcoal canister system that stores fuel vapor and allows it to be draw into the engine under certain conditions instead of being vented to the atmosphere (where it becomes harmful to the atmosphere). Evaporative emission problems can be anything from a loose gas cap to problems with the vacuum switching valves (VSV). Toyotas that had the charcoal canister under the rear of the vehicle incurred problems after exposure to salt and water on the roads during the winter.
The most likely culprit: the VSV valve located on the charcoal canister is sticking, just above the spare tire. Its a real pain to get to, especially if the little screw is rusted. A new vsv will run between $40 and $120. P/n is 90910-12198. Just replaced mine, and it fixed the P0446 code.