P0446 TBP test failure. How can I figure out for sure if its a faulty valve or canister?

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Jesse Goldman, Aug 3, 2021.

  1. Jesse Goldman

    Jesse Goldman Junior Member

    Joined:
    Jul 9, 2021
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    Location:
    Springfield MA
    Vehicle:
    2007 Prius
    Model:
    Five
    Been getting check engine light with P0446 code on and off for a few weeks. I reset the code and then it will come back after maybe 100 miles. I know some people say tighten the gas cap but I have tried that and don't think its the issue. Took it the dealership and they said "tank bypass or vent valve not working." then I asked for clarification and they said they meant to say "and" not "or". So they wanted to replace the charcoal canister and filter for a total of $1k after parts and labor. I know the charcoal canister is like $300 and the valve is much cheaper. So is it possible to find out myself if the canister really needs replacing, or if it's just the valve? How does Toyota figure this out? I have miniVCI and techstream on my laptop and I tried getting more info but it doesn't say much. It says it failed the TBP test, and something like it's not meeting the minimum pressure requirement to pass . I attached pictures of the health check results in this post. I just want to find out exactly what's causing the code and then buy the parts myself and take it to a cheaper mechanic. I would love to do it myself but I cant find too many guides on how to replace canister and valve on a Prius and I have never done any car repairs before, don't have any tools for it really.
     

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    #1 Jesse Goldman, Aug 3, 2021
    Last edited: Aug 3, 2021
  2. bisco

    bisco cookie crumbler

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    Plug-in Base
    you might need the service manual and troubleshooting tree to go with the codes and subcodes
     
    #2 bisco, Aug 3, 2021
  3. pasadena_commut

    pasadena_commut Senior Member

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    Vehicle:
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    This is a generic suggestion, as I have never worked on this part of a Prius.

    Do not be so hasty in ruling out the gas cap. They can have cracks or small holes in rubber gaskets which you might not see but which leak enough to throw a code. Does your state have smog check? See if one of those shops will test the gas cap for cheap/free. If that isn't an option install an inexpensive replacement, reset codes, and see if it comes back. A generic should be fine - we recently changed one on an Accord with a Gates brand cap off ebay for $10 and it did the trick.

    Also, check any hoses in and around the canister and the solenoid. My son had a bad gas smell in his RV and thought it was the canister, but no, the hose connecting it to the solenoid had rotted out leaving a big hole. Rubber hose is cheaper than components!

    If you want to test the solenoid it isn't hard if you can pull the part off the car. Get a bike pump with a conical adapter and stick that into one hose into the solenoid and apply a little pressure. (I have seen people blow into one of the hoses, but those things are always filthy, so it isn't going in my mouth.) Nothing should come out the other end. Then apply 12V to the two pins (which is all these usually have) and the same pressure should hiss as it runs through the device. The solenoid should also click when 12V is applied and removed. I keep an old PC power supply modded so that it starts up when AC is applied to supply the 12V. You could also use wires from the car battery, but if you do, a small fuse should be in the positive lead near the battery terminal, because if the device is shorted, or you cross the two wires, the car battery can supply a lot of current and the wires will rapidly turn red hot. This PC power supply has some internal safety circuitry, so it shuts off if the 12V is shorted, and it comes back after AC power is removed and reapplied.
     
    #3 pasadena_commut, Aug 3, 2021
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