P0455 is a large EVAP leak.
There are two valves that help seal the system for diagnosis.
The EVAP vent valve is near the back of the car... it is normally OPEN and allows the fuel tank to vent to prevent pressure or vacuum build up with expansion from heat/etc. It vents through the charcoal canister to absorb any fuel vapor smell before actually leaving the system.
The purge valve is under the hood, it is normally CLOSED... meaning not allowing anything to flow through it. One end is attached to intake manifold(vacuum), the other end goes to the fuel tank.
So under normal operation, the purge valve is closed, and the vent valve is open.
When the EVAP leak test runs, it monitors the fuel tank pressure sensor.
When it runs, the ECM will command the vent valve shut, then it will start pulsing the purge valve anywhere from 10-100% duty cycle to start pulling a vacuum on the system. While it's pulling vacuum it is monitoring tank pressure(negative pressure in this case, vacuum)...
Once it reaches the desired level of vacuum, it will stop pulsing the purge valve, so the system should technically be 100% sealed now.
Now the rate of vacuum decay(how long it takes to go back to atmospheric pressure) is measured. If it loses pressure very slowly, it will set the P0442(small leak). If it loses it very quickly(or isn't even able to pull a vacuum because it leaks so quickly... OR if the purge is not functioning), it will set the P0455(large leak) code.
If you don't have access to a small handheld vacuum pump with some adapters and stuff, OR a reasonably capable scan tool(that can read the fuel tank pressure sensor voltage or at least purge duty cycle in real time), then you really can't do a whole lot easily. I highly recommend at least getting a cheap vacuum pump w/ a gauge... they're under $20 at harbor freight I'm pretty sure.
The more involved way is to take parts off and bench test them.
To bench test the purge valve, remove it and see if you can blow through it. You shouldn't be able to. You also need a 12v battery and some wire and steady hands. Now you need to jumper the pins on the purge valve. Polarity doesn't matter, it's just an electro-magnet and doesn't care. If you jumper it and you hear a nice loud, solid click, it's probably functioning fine.
Now if you have a vacuum pump, you can go a step further and pull a vacuum on the purge(do it on the manifold vacuum side... it won't hold on the tank side). It should hold 17in-Hg easily and not drop off. Now when you jumper the pins, the vacuum should drop immediately.
Onto the EVAP vent valve...
http://www.jimellismazdaparts.com/showAssembly.aspx?ukey_assembly=1149502
#18-741C
It is normally open, so you should be able to blow through it. Now apply power and ground to it's pins, and you should no longer be able to blow through it. Opposite of the purge, so much more awkward to bench test.
It could potentially be either one, or could be a gaping giant hole in the fuel tank.
The way I test it at work is to use a scan tool to command the vent valve shut, and hook up a smoke machine somewhere between the purge valve and the fuel tank. Takes no more than 15min to figure it out as long as the leak isn't coming from anywhere hard to access.
If you're going to take a wild guess for the next part... I'd start the car up and pop the hood, find the purge valve under the hood and listen for it clicking and operating. It should be very noticeable. They do function quite a bit during normal driving, and normally somewhat regularly when the car is warming up and all the emission self checks are being started. If you feel it clicking/operating then I'd probably take a guess that the vent valve has failed.
