Wednesday, June 12, 2013

Nissan Altima P0446 Evaporative System Canister Vent Control Valve Circuit

This post describes the diagnosis and repair for the P0446 code on a 1999 Nissan Altima. This applies to all second generation Nissan Altimas which are 1998, 1999, 2000, 2001.

This website does a great job describing the function of the emissions system and the roles of the often confused VENT solenoid and PURGE solenoids. Below is my stab at a high level summary.

Basically speaking, the evaporative system on your car is used to route vapors from your gas tank into the combustion chamber to lower the ignition temperature to reduce NOx emissions. The VENT solenoid is used to control the flow of vapor into the combustion chamber. The PURGE solenoid is used to purge the excess vapors from the system and in the case of some manufacturers create the vacuum. The evaporative emmision control system throws a code when vacuum/pressure is not able to be maintained, which means there is a leak one of the lines or one of the two solenoids or the EGR valve is not working.

If that sounds like a lot of things can go wrong with this code, it is. You can chase this code for quite some time but there are three easy diagnostic tests that you can use to at least rule out components (or in my case find the problem!).

1. Check/Replace Gas Cap - one of the most common and easily fixed causes for the P0446 code is a loose or defective gas cap. Without a tight sealing gas cap the evaporative system cannot pull a vacuum to extract the vapors from the gas tank. I recommend first checking to see if your gas cap is tight and clearing the code  If it comes back after driving, you might want to replace it for a few dollarsclear your code again. If it still comes back, move on to diagnosing other components.
2. Check/Replace the EGR Valve - this valve often gets caked with carbon deposit causing it to either seize or prevent it from holding a vacuum.
3. Check/Replace the Vent Solenoid Valve - because of the design (exposed housing, minimal sealing) and placement (low and behind a wheel) of this solenoid valve it is an extremely common/recurring failure point in the Northeast due to corrosion in the evaporative system. This ended up being what was wrong with my system.

Hopefully this helped and one of those three problem parts ended up being the source of your check engine code. If not, please post what ended up being your problem/fix so I can add to this post!

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