P0446 Evaporative Emission Control System Vent Control Circuit Malfunction
The evaporative emission control system checks to make sure there are no leaks in the lines that run from the gas tank to the charcoal canister and up to the engine. It also checks to make sure the gas cap is sealing correctly.
When you fill your car with gas, the vapors in the tank get forced into a canister filled with activated charcoal. Also, on a hot day as the gas heats up and vaporizes, those same vapors push into the canister where they’re stored. But the charcoal can only hold so much vapor. At some point it has to be emptied. The emptying process is called “canister purge.” Here’s how it works.
The computer orders a canister purge by powering open a purge solenoid. That opens the vacuum line between the canister and the intake manifold. At the same time, it opens a VENT solenoid. That allows fresh air into the canister. So the engine is literally sucking out the gas vapors and purging the canister with fresh air. The computer has to adapt its fuel strategy to take advantage of all the extra gas vapors coming into the engine.
The computer notices the canister is empty when it detects a leaner condition (all the vapors are used up) and it resumes normal fuel delivery. Some car manufacturers then close the VENT solenoid but leave the PURGE solenoid open. That creates a vacuum throughout the entire fuel storage system. Once the correct vacuum is reached, it closed the PURGE solenoid and waits to see if the vacuum holds. If it doesn, the system passes the test. If it detects a leak, it sets a code.
The computer also watches the electrical activity during the testing process. When the computer operates the solenoids, it sees a voltage drop on the line. If it doesn’t see a voltage drop, it knows there’s a problem with the electrical–either the wiring is broken, or the solenoid isn’t working. In this partcular code P0446, the computer has detected an electrical problem with the VENT solenoid.
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© 2012 Rick Muscoplat
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