The P0456 engine code means that the pressure has decreased in the Evaporative Emissions System (EVAP system), which is potentially caused by fuel vapors escaping the fuel tank and entering the atmosphere. The Powertrain Control Module (PCM) is responsible for monitoring the fuel tank pressure (FTP). A drop in FTP points to a loss of pressure in the EVAP system, which causes the PCM to activate the P0456 code.
There are no other symptoms that fuel vapor is escaping from the fuel tank other than a malfunction indicator lamp that appears on the dashboard of the vehicle once the PCM activates the P0456 code. The most common reasons why the P0456 code is activated are a worn seal on the gas cap or an insufficiently tightened gas cap. This code may also appear if a car is being fueled while the engine is running. In both instances, the fuel tank is exposed to the outside, which allows gasoline vapor to escape from the gas tank.
A much more serious reason for the P0456 code activating is a leak in one of the hoses that run from the fuel tank or the EVAP system. Another potentially serious problem is that the EVAP system canister itself is leaking.