Rick's Free Auto Repair Advice

P0446 Fix

Fix for 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. One common trouble code is a P0446 Evaporative Emission Control System Vent Control Circuit Malfunction

What the vent solenoid does

When you fill your car with gas, the fuel vapors are absorbed by the charcoal canister. Later, after you start the engine, a purge valve opens and engine vacuum sucks the fuel vapors out of the canister. At the same time, a vent valve opens to allow fresh air into the canister.

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 closes the PURGE solenoid and waits to see if the vacuum holds. If it doesnt, 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 particular code P0446, the computer has detected an electrical problem with the VENT solenoid.

GM has experienced many problems with insects building nest in the vent solenoid, preventing it from properly venting. When that happens, GM recommends rerouting the vent hose and installing a screen filter.

However, GM vehicles can also set a P0446 code if the fuel tank pressure sensor is faulty.

How to troubleshoot a P0446 on a GM vehicle

You’ll need a scan tool with live data. With the engine off, check the reading for the fuel tank pressure sensor (FTP). With the engine off, the FTP should read 0. If the FTP reads a positive value, remove the gas cap to relieve any fuel tank pressure. Then read the FTP reading again. If it still reads a positive value, the FTP sensor is bad and must be replaced. It’s a bad FTP and not a bad vent valve in this case. Unfortunately, you probably have to drop the fuel tank to access the FTP sensor.

fuel tank pressure sensor on top of fuel tank

To learn more about EVAP systems, read this article

 

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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