I had a P0456 code come up a couple of months ago. I searched here to see what might be wrong. I replaced the gas cap with a new OEM cap. The code came back. I checked all of the hoses and couldn't see anything that looked cracked, etc. Pulled the panel under the fender to check for anything obvious by the leak detection pump. Nothing I could see.
I took it to a local shop and they did a smoke test. They said they couldn't find any leaks but *maybe* the leak detection pump was malfunctioning. I had them replace the pump. About two weeks later the code came back. The shop thought the leak detection pump might be bad and replaced it (no charge). A few weeks later the code came back.
I took it back in today. They checked it and are stumped. They did another smoke test and said it shows no leaks at all. They cleared the code for me but aren't sure where to go from here. They did say that the smoke test pressurizes the system and the vehicle puts it under vacuum to test. This is a good shop that I have used in the past.
Anyone have a similar hard to diagnose P0456 issue? I haven't done anything like remove the gas cap with it running, overfill the tank, etc. Also, I haven't experienced any issues - it has been running great and my mileage has been fine.