What does it mean?
In spite of the fact that fuel is delivered in liquid form, the internal combustion engine runs, not on liquid fuel, but on fuel vapors. This works out rather conveniently, since liquid fuel is far more easily manageable, but easily vaporizes into usable form, even when it’s below zero. The tendency of fuel to vaporize, called volatility, makes for good engine starting, performance, and efficiency. At the same time, fuel volatility creates another problem, evaporative emissions.
Fuel vapors are carcinogenic and environmentally destructive, which is why fuel storage, at the refueling station on in your vehicle, is designed to significantly reduce evaporative emissions. In your vehicle, the evaporative emissions control system, usually just referred to as the “EVAP” system, keeps fuel vapors from escaping into the atmosphere. Using a system of tubes and valves, the EVAP system routes fuel vapors from the fuel tank into an activated-charcoal canister for storage.
At certain times, when the engine is running, the EVAP system will route fuel vapors from the canister to the engine, where they are burned. The ECM (engine control module) controls the EVAP system valves, monitoring system function via pressure sensors. Many of the self-tests that the ECM performs on the EVAP system require that certain conditions be met first, such as sufficient battery voltage, specific fuel level, and specific atmospheric temperature.
Aside from circuit tests, one of the first things that the ECM needs to verify is that the fuel tank and the rest of the system is sealed. To run this test, the ECM draws a vacuum on the tank. If the ECM detects that the EVAP system is unable to produce or hold a vacuum in a certain amount of time, it records a diagnostic trouble code (DTC) in memory, P0440, “Evaporative Emissions (EVAP) Control System Malfunction,” and illuminates the MIL (malfunction indicator lamp).