Sorry for the delay (phone call).
The P0440 indicates absolutely zero response from the heart of the evap leak system, the natural vacuum leak detector (NVLD), which can go a few ways.
The evaporative system involves the vapor portion of your fuel system and stretches all the way from the gas cap to the engine. The way it's tested goes like this...
At key-off, the NVLD closes automatically, which seals the evap system to prevent vapors from escaping to the atmosphere. Because the gas law
states pressure within a sealed vessel will decrease as temperature drops, the NVLD was designed to sense this pressure drop and close a switch that notifies the engine controller (PCM) of the act. If a few days pass without seeing a pressure switch state change, the PCM's leak monitor will set a P0456 code. This is either an indication of an actual leak or possibly a failure of the NVLD itself... things are uncertain at this point.
Now that the P0456 has been set, the leak monitor goes to work with invasive tests that will give it a better idea just how large this (supposed) leak really is. At this point it doesn't know.
While driving, the leak and purge flow monitors will become active, actively drawing vapors from the evap system while looking for a state change from the NVLD. This is the same thing that happens overnight with the passive test I described earlier, only it can be done quicker and more often in this manner.
If it STLL can't get the switch state to change, the P0440 code is set. This could indicate anything from the gas cap being completely left off to a very large leak in an evaporative line, to outright failure of the NVLD. A leak test of the evap system would be needed first to see if any leaks were present... and then if none are found, replace the NVLD.
The evaporative lines on your Pacifica are very durable and seldom have issues with leaks, so your choices effectively come down to a lost gas cap, bad NVLD or a smashed vapor canister (somewhat exposed ahead of tank). Wiring issues can't be excluded, as the NVLD pigtail is somewhat exposed to road hazard dangers and may have a broken conductor even if the wire appears intact.
If history repeats itself (and it does with evaporative failures), you'll be needing an NVLD. They're sometimes available separately, but often they come with a vapor canister assembly, which drives the price up a bit. You'll find the NVLD/ vapor canister assembly mounted ahead of the tank on the left side underbody of the vehicle.
The fault will clear on its own after the monitor sees 2-3 "good trips" (non-failures), but you may elect to perform a battery
disconnect to speed things up a bit. Either way, you'll know in three days whether the repair was successful.
If you have any questions, don't hesitate to write back. Be glad to help!