That is a very generic code and it's not listed in the manual. It means you have a large leak in the system.
But here is what I was able to find out
Causes:
Missing Fuel Cap
Defective or damaged Fuel Cap
Distorted or damaged Fuel Tank filler neck
Torn or punctured Evaporative System hose(s)
Defective Fuel Tank Sending Unit gasket or seal
Split or damaged Carbon Canister
Defective Evaporative Vent Valve and/or Evaporative Purge Valve
Defective or damaged Fuel Tank
Defective Fuel Tank Pressure Sensor
Description:
The Code P0440 indicates that there is a large leak in the Evaporative System, but this is somewhat misleading. What the code really indicates is that the Evaporative System will not create a significant vacuum when it performs its leak test. Let's go over a little about how the Evaporative Leak Test is performed by the Power Train Computer. When the leak test is performed, the vehicle must have been sitting for at least 4-8 hours in order for the engine temperature and air temperature to be identical. This is to provide a base line for the test because gasoline and diesel are volatile fluids that expand and vaporize easily with warm temperatures. When the Leak Test initiates, the Vapor Canister Vent Valve is closed to prevent any fresh air from entering the Evaporative System. Then the Purge Valve is opened which allows the engine to create a vacuum in the Evaporative System. After a specified time interval, which is usually about 10 seconds, the purge valve is shut off and the vacuum level in the system is measured with the Fuel Tank Pressure Sensor. Then a count down initiates which measures the rate at which the vacuum decays in the system. If the vacuum decays faster than the specified rate or, if only a small amount of vacuum is reached on 2 tests in a row, then the Power Train Computer will fail the Evaporative System for a large leak and set the code P0440. Now, what are some of conditions or components that will cause the Evaporative System to fail its leak tests?