Code P0455?

A P0455 is a large EVAP leak. You don't really need a better scan, just the tools to find the leak. The first thing I would do is check the purge solenoid on the engine end ( image 1), if it's stuck open you can get this code, but usually that one will also make the engine hard to start when you fill up with gas. Testing is simple, one hose goes to the intake manifold for vacuum. The other goes to the fuel tank. Remove the second line and start the engine. Hear a vacuum leak? Put your finger over the hose fitting, feel vacuum? Unplug the electrical connector, if you hear or feel vacuum, it's stuck and needs to be replaced. Next test, go under the vehicle and locate the charcoal canister and its vent valve (image 2). It should be open, but if you unplug it and apply battery power to the two pins it should click shut. With it shut it should not pass air. If it's stuck open it will cause a large leak and that is the common failure mode with those valves. If your tool can read any live data and one of the pids is fuel tank pressure or pressure sensor voltage, you can test this by applying voltage to the solenoid in the rear and in the front. That would be the same as the purge & seal test the OE tool does, if the pressure sensor, then shows a drop as the vacuum pulls the tank clean, you then disconnect the power to the front valve. It should hold the vacuum for a long while, if not you have a leak. Clamp off the line coming from the tank valve to its vent and test again. If it holds now the vent solenoid is leaking. If you have access to a smoke machine or similar you can also use it to find leaks in the lines and hoses.

Doesn't really matter as those parts are not affected by fuel level. The system, however, won't normally run the EVAP test on a full or empty tank, most are set up, so they test between 1/4 and 3/4 tank. Nope, it shouldn't do much if it's only an EVAP problem.

That can be common if the leak was the purge solenoid. They can stick open, set the code and you hit a bump and it frees the valve. Until the next time it sticks. A P0455 could lower your mileage if it was the purge sticking open or if the leak was in a spot where fuel could find it's way out. Like a fill pipe or vent tube, but usually you would notice those as the outside of the car would stink like gas all the time from a leak like that. A more common cause of mileage changes would be the fuel itself. The higher alcohol content that is being pushed (15% instead of 10%) is going to reduce mpg in any vehicle that isn't a true flex fuel.