The fuel tank must be checked for pressure loss to comply with a relatively new standard that of course applies to CA to a higher degree first before the rest of the country.
There is a charcoal filled canister to absorb fuel out of escaping vapors from tank while it is parked and while it is being fueled.(EVAP canister)
There is a valve that is normally open to allow fresh venting of fuel stripped vapors out or to allow fresh air to enter while vapors are being drawn through canister to be burned in engine where they can be rendered harmless by combustion and subsequent catalytic converter actions.(canister shut-off valve)
There is a pressure sensor to monitor pressure in tank during a leak test.
And there is an electric valve that is pulsed to varying opem time periods to allow the engine manifold vacuum to pull the vapors from canister (EVAP valve).
When you have driven a qualifying distance and if there are no fatal trouble codes in fuel computer (existing codes that would make test falsely fail) then the test is carried out when you idle uninterrupted for 30 seconds.
First the shut-off valve is closed and pressure is checked for stability, if fuel is hot and excessively vaporous the press will rise and test will not commence, ohterwise initial press is noted. Shut -off valve is depowered and opens up.
The evap valve is operated and press is checked for no change, if it goes down in press then can shut-off valve is stuck shut and test ceases with a code for shut-off valve otherwise valve continues operation.
The can shut-off is closed again and a vacuum should now occur in tank and associated lines within a reasonable time (8 seconds +/-), if not a code is set for big leakage otherwise the evap valve is shut off and vacuum is held at some level.
The press is monitored by fuel computer for 6 seconds+/- and if it holds test is over for this driving cycle with no codes. If vacuum is lost then a code for small leakage is set.
Any one of these possible single failures must have occurred for a second time before your CE light will come on.
The allowable leakage for failure is low.
The part that is killing the techs is that many things that are not actually leaks may set the code.
Evap valve having low flow won't pull a high enough vac and a large leak code can set.
An obstruction in the canister or fuel tank fitting will not allow the vac to be pulled on those during the evacuation phase and thenthe vac goes down slow during the monitor phase and a small leak code will set.
Of course any small leaks will also cause code to be set accurately.
Be patient with dealer and invest in a pack of chewing gum to put in front of CE light till they can prevail over the trouble.