What does OBDII code P0446 Evaporative emissions control vent circuit low mean?

The previous answer was good for high dust environments (and if you have a Ford Van all environments are to be treated as high dust) but for those of us in high corrosion environments, consider the following:

If you have an Explorer (Or Ranger/Mazda B-series) check for a broken wire near the left side framerail near the rear axle.

If you have a Hyundai (including Kia), Nissan (including Infinity) or Toyota (including Lexus or Scion) you should disconnect the electrical connector and then apply and remove 12V power to the vent solenoid/valve. The difference between 12V applied and not applied should be the difference between not being able to blow through it and being able to. After testing at least 5 times, if it fails even once, replace the solenoid/valve.

In the case of Nissan or Toyota, if replacing solenoid/valve it is usually best to replace the canister as well as you might inadvertently break the canister while replacing the solenoid/valve. With many Nissan vehicles, it is almost impossible to not break the canister without using techniques involving unreasonably high fire hazard.

Hey Brian==It is probably the gas cap not on tight. Have the codes removed and see if it comes back on after you tighten the cap.GoodluckJoe

Well, my experience with a 2000 GMC 3/4 ton truck says otherwise.

A loose gas cap will not give you P0446 because that is the code for a failed vent test.

Here's how it works: To check the vent system the engine vacuum valve is opened at the same time as the charcoal filter vent valve is closed. This creates a vacuum in the gas tank and when the vacuum reaches a certain level, the vacuum valve closes and the vent valve opens and a timer is started. If there is a restriction in the vent valve or it doesn't operate electrically, the tank fails to re-pressurize within the correct time and you get a P0446.

Here's what I did: I removed the vent valve and found it choked with a fine dust. I cleaned the valve, but noticed that there was lots of dust in the tube running down to the charcoal cannister and realized the gravel filter in the vent valve is useless at stopping dust. I took out the cannister and discovered an amazing amount of dust clogging it up. I cleaned the cannister by holding it vent side down and sharply tapping it many, many times it while blowing air (at low pressure) through it toward the vent side until it passed air freely both ways. Did I mention that there was an amazing amount of dust that came out of the cannister? I would think that gasoline residues from overfilling might clog up the cannister too, but that didn't happen in my case.

Below are what I consider the possible causes of P0446 in the order of least likelihood (IMO) for a vehicle used in a high dust environment:

Electrical vent driver circuit in computer burned out (especially if the vent valve is shorted), electrical vacuum valve driver circuit burnt out, vent valve coil open or shorted, vacuum valve coil open or shorted, bad vacuum sensor in the gas tank, vent valve filter clogged with dust, charcoal cannister clogged with dust or residues.

Removing and replacing the vent valve and especially the charcoal cannister in a GMC truck is a major, major pain in the a-- because the entire vehicle was carefully engineered by GMC so as to make replacement as difficult as possible. It does no good cleaning and replacing a clogged vent valve as the cannister will be clogged too. A new cannister is about $175, but a cleaned one is free.

BTW, my vent test works perfectly now. Added by Protoart: If you have the P0446 code and the vent valve is bad, don't continue to drive, or you will clog (saturate) the canister with gas for sure. My wife's 05 Scion, check engine light w/ P0441,0442, and 0446.......cleared with my scanner, never came back after I tightened the gas cap