I meant to say evap purge valve solenoid, and canister vent solenoid. They are kind of like a relays, same concept with magnetizing but they control the flow of air. Maybe I shouldn't call it a relay, ha!. The ECU turns them on and off with a transistor to create a vacuum in tank (stupid emissions, vapor gas bullshit). The pressure sensor is part of it. Hence, when you don't tighten gas cap, it will throw a 1456 code.
1456 deals with the tank side of things, 1457 deals with the front. They both may come to the same conclusion but testing vacuum is important to see what's failing.
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But once you identify what's not working. You test that solenoid, and if that's working, you look at ECU. I identified that the solenoid was working then trace the circuit to identify the transistor. Once, Identified, you can use a multimeter to test it. I believe the diode settings. See here:
https://www.youtube.com/watch?v=2IkAPU9X33k
The service manual flow chart does show you how to test each and every section but I'm not sure about tank side (1456) . I think they are just sensing negative pressure? I have to look into it. You probably have to vacuum the whole thing.
You can identify the transistor by the markings on the transistor. Yours's look the same as mines, "D1780." Look up SD1780 on eBay. They label them "Honda SD1780 transistor." Alternatively, Digi key or other transistor suppliers. I thought buying from eBay is faster but it's not.
I do have a spare if you want it.
And sorry if I didn't explain it very well. I am happy to get on a conference call to help.
Thx
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The previous owner, also mentioned that they replaced the solenoid too. Looked up a replacementment and then picked one up on ebay! 
