Of all the things that can malfunction on your truck, its evaporative emissions system is probably both the most infuriating and the most insidious. Gasoline is an extremely unstable liquid, especially when it's warm -- without some way to seal the system, gas would quickly evaporate out of the tank and cost you big bucks at the pump. Code P0446 indicates a problem with one of the solenoids in this system and may be warning you of a problem that only your wallet is going to notice.
Check for all system malfunction codes that can help to track down the specific problem. This code translates as "Evaporative Emission Control System Vent Control Circuit Malfunction," which itself means that the solenoid or the circuit (meaning the solenoid and the air tube system) that allows air to flow into your charcoal canister has malfunctioned. You may find other related codes like P0447, P0448 or P0449 that can help you to figure out exactly what's malfunctioned in the system.2
Locate the EVAP vent valve solenoid. The EVAP vent valve allows air to flow into your charcoal canister. If the vent solenoid sticks open, the system will fail to pressurize and send fuel vapors back to the tank through the purge line. This solenoid has a two-prong connector; the pink wire supplies constant positive current from the battery and the white wire supplies a ground signal from the control computer. You can find the vent solenoid in one of the rubber lines exiting from the charcoal canister at the front of your truck. Locations vary depending upon the engine option, so follow each line to its terminus until you locate the vent solenoid.3
Check the vent solenoid line for cracks or kinks and check to ensure that the ends of the vent hose connect securely to the solenoid and to the charcoal canister. Check the harness connector at the vent solenoid and make sure that it's tightly connected to the solenoid. Loose, corroded or missing connector terminals will break the circuit to the solenoid and trigger a failure code.4
Turn your ignition key to the "on" position, and unplug the vent solenoid harness. Connect one end of your 12-volt test light to the battery's negative terminal and touch the sensor probe to the chassis-side terminal corresponding to the pink wire. This wire gets a constant supply of power, so the light should illuminate then you touch it to the terminal. If it illuminates and the vent system passes all other check, replace the vent solenoid. If it doesn't illuminate, then your problem is in the computer or a sensor in the EVAP system.