OBD-II Code P0455 is defined as a Evaporative System Malfunction, Gross Leak
Get it diagnosed by a professional
The evaporative control (EVAP) system captures any raw fuel evaporating from the fuel storage system (e.g. the fuel tank, filler neck, and fuel cap). Under precise operating conditions—dictated by engine temperature, speed, and load—the EVAP system stores and purges these captured fuel vapors back into the combustion process.
The EVAP system is designed not only to capture, store, and purge any raw fuel vapors that leak from areas in the Fuel Storage system, but also to run a series of self-tests that confirm or deny the operational and vapor holding ability of the system. This is an important task because at least 20 percent of vehicle-produced air pollution originates from malfunctioning Vehicle Fuel Storage systems.
There are many ways to "leak test" the EVAP system, but most perform the leak test when the vehicle is sitting (like over night) or during the initial start-up after the vehicle has been sitting over night. The EVAP system's operational performance is also tracked by the Powertrain Computer by reading the change in the oxygen sensor voltages and short term fuel trim whenever the stored vapors are released or "purged" back into the combustion process. These values should indicate that fuel is being added to the system and that the overall mixture is getting richer. The purging process occurs when the vehicle is under acceleration, which is when most vehicles require additional fuel.
The P0455 code indicates that there is a large leak in the EVAP system, but this is somewhat misleading. What the code really indicates is that the EVAP system will not create a significant vacuum when it performs its leak test, as monitored by the Fuel Tank Pressure Sensor.
Here is how the evaporative leak test is performed by the Powertrain Computer:
Retrieve the code and write down the freeze frame information to be used as a baseline to test and verify any repair.
Perform a pressurized smoke test. During the test, perform a careful and close examination of the visible hoses, fuel filler neck, installed filler cap, fuel tank, vent valve, purge valve, and vapor holding canister. Open the Throttle Body to make sure there isn't an internal leak that is flowing smoke into the intake manifold. (Be sure to close off the vent valve during the smoke test! If possible, use tape so you don't overwork the electrical portion of the Vent Solenoid by having it energized for too long.)
Run an additional smoke test while using the scan tool live data stream feature with the Fuel Tank Pressure Sensor PID in plain view. As the test inserts smoke into the fuel storage system, the Fuel Tank Pressure readings should increase. If the pressure readings do not increase, the system will think that no pressure or vacuum is being created when the EVAP monitor is performed when, in fact, there is a pressure/vacuum being created that Fuel Tank Pressure Sensor is unable to read. The Fuel Tank Pressure Sensor is the primary feedback sensor that the Powertrain Computer relies on for the leak test data each time the EVAP monitor is run.
Inspect and test the fuel cap to determine how well it fits onto the Fuel Tank Filler Neck. If the cap will not seal or hold vacuum/pressure, then it can trigger the P0455 code.
Verify that the Purge Valve and the Vent Valve work properly and hold vacuum for a sustained amount of time—at least thirty to sixty seconds. If either one of these valves function improperly, the system will not develop and/or hold the proper amount of vacuum. You may have to remove and bench test them. Also be sure to measure the electrical resistance of the solenoids to be sure they are in spec.
If all the components seem to function properly, then perform another smoke test of the entire EVAP system, but this time, use your sense of smell. Go around the entire system to see if you can smell any fuel odor. In some cases, the smoke will exit in a manner that is invisible, but there will be evidence of a fuel odor that will lead you to the problem area. This area may be completely hidden by the frame, fuel tank, etc.
If all tests fail, clear all the codes and perform a drive cycle test drive to make sure that the code re-sets are what are the freeze frame data points are referring to.