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98 Honda Civic Ex. For a while now my car will throw a p1456 evap code but sometimes it doesn't and the system completes the test.

I'll randomly hook up my obd2 scanner and it'll either have a pending code for p1456 or not.

It seems to pass the test if outside ambient temps are 70F or higher. If it's colder out I'll most likely see the code come up.

Paid a shop to run a smoke machine through it but there were no obvious leaks. As per suggestions I've replaced the two way valve and bypass valve but didn't make a difference.

12v on both the vent and purge valves gets them to activate.

My car doesn't need to pass emissions since it's older than 15 years but I don't like having the check engine light on all the time.

Things replaced: Gas cap, filler neck, bypass and two-way valve

Edit: Pic of the sending unit area enter image description here

Any suggestions?

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  • I find it somewhat interesting that you didn't mention the gas cap at all, i.e. have you replaced it? Have you even considered it as the cause?
    – juhist
    Jul 13, 2017 at 18:14
  • @juhist Just edited and included things I've replaced. Gas cap was first one
    – user140123
    Jul 13, 2017 at 18:16
  • Have you checked the seal on the sending unit? It's fairly common for the tank to rust and cause a poor seal around the sending unit. This may or may not show up on a smoke test depending on how bad it is.
    – Ben
    Jul 13, 2017 at 22:05
  • @Ben I have not. Would this explain why in hotter ambient temps the evap tests pass? (Seal expanding?) Still trying to use that little clue to help pin point the problem
    – user140123
    Jul 13, 2017 at 22:35
  • Yes, that would be my guess, based on the information. I don't remember if there is an sending unit access port on these Civics but it's worth taking a look.
    – Ben
    Jul 13, 2017 at 22:44

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