I'll start from the beginning.

~ February 2012, I went to Infiniti dealership still under warranty because of a SES light on. The code was P0456 - "Small evap leak detected". They told me to replace the gas cap. I did. I got busy, warranty ran out, light kept coming back on. I came back ~6 months later, they still honored it as a continued warranty claim, and performed a smoke test on my EVAP system- found no leaks. They told me to replace my gas cap with an OEM gas cap, not the new one I purchased. I obliged. Another 6 months go by, light keeps coming on like every 2 weeks, but I feel like I'm way past my warranty expiration to do anything about it. So I don't. Now I'm sitting here with this car that the SES light comes on about every 2 week. Sure, my own fault. Let's move past that.

Now that I've been dealing with this for so long, I can tell you that it doesn't happen when the temperatures drop. It's only in the heat. In fact, if it's 50 degrees one day, 75 the next, I can almost guarantee the light will come on. It won't come on all winter (I'm in the midwest). So that makes me think something to do with pressure. BTW, here's the Infiniti service bulletin related to the issue THAT INFINITI SHOULD HAVE FOLLOWED ON DAY 1 - http://www.infinitig37.com/TSB/1SE28.pdf.

TL;DR - SES light, P0456 comes on. Typically in warmer weather, definitely with increases in temperatures. Infiniti performed smoke test, found no leaks.

So, what do you guys think it is? Can I fix it myself? Any idea on the costs?
2009 G37x coupe.

 

if they used a smoke machine to pressurize the evap and fuel system and found no leak i would suspect there may be a problem with the fuel tank pressure sensor. the sensor may be providing false readings to pcm.