What does it mean?
Gasoline is a toxic substance, which has numerous health and environmental effects that are less than desirable. Exposed to the atmosphere, gasoline would evaporate and pollute the air. To prevent your vehicle from causing undue air pollution, the entire fuel system, from the gas gap to the fuel injectors, is sealed. Still, faulty vehicle fuel storage systems may account for up to 20% of overall vehicle emissions!
During normal operation, such as running the engine (pulling fuel out of the tank) or refueling (putting fuel in the tank), a perfectly sealed system wouldn’t work very well. The function of the evaporative emissions control system, typically referred to as the EVAP system, is to store and route fuel vapors from the tank to the engine, without allowing them to escape into the atmosphere.
Apart from the fuel tank and gas cap, the EVAP system typically consists of valves, pressure sensors, flow sensors, a canister of activated carbon, and tubing from the tank to the engine. Some vehicles use a vacuum pump for engine-off EVAP system testing. Depending on specific operating conditions, the engine control unit (ECU) opens and closes valves to route fuel vapors to the engine, where they are burned.
At other times, depending on the vehicle, the ECM will test the EVAP system to make sure it is sealed from the atmosphere. If the ECU detects a fault in the EVAP system, it will set a DTC (Diagnostic Trouble Code) and illuminate the MIL (Malfunction Indicator Lamp). Specifically, DTC P0455 refers to the ECM having detected a large leak in the EVAP system, and it may be accompanied by similar leak codes, such as P0442, P0456, or P0457, or perhaps a general EVAP system fault code, P0440.
EVAP problems don’t typically manifest themselves as drivability problems.