P0455 OBD-II: Evaporative Emission System Leak Detected (large leak)

What Does OBD-II Fault Code P0455 Mean?

OBD-II Code P0455 is defined as a Evaporative System Malfunction, Gross Leak. P0455 appears when the EVAP system has determined that it cannot create enough vacuum pressure in the system, which is typically caused by an air leak. The most common reasons for a P0455 OBD-II code are a missing or loose fuel cap, damaged fuel tank filler neck, damaged EVAP system hoses, a damaged fuel tank, or another issue causing a leak in the EVAP system.

What are the most common symptoms of a P0455 Code?

• Check Engine Light will illuminate
• In most cases, there are no adverse conditions noticed by the driver
• In some cases, there may be a noticeable fuel odor caused by the release of fuel vapors

What are the most common reasons for a P0455 code?

• Missing, 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

Common Misdiagnoses

• Missing or loose fuel cap is easy to check and replace, but the problem may be deeper in the fuel system.
• Evaporative Purge Valve
• Evaporative Vent Valve

Get it diagnosed by a professional

Find a shop in your area

Polluting Gases Expelled

• HCs (Hydrocarbons): Unburned droplets of raw fuel that smell, affect breathing, and contribute to smog

How does an EVAP system work?

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.

P0455 Diagnostic Theory for Shops and Technicians

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:

1. When the leak test is performed, the vehicle must have been sitting for at least four to eight hours so that the engine temperature and outside air temperature are identical. There must also be between 15 and 85 percent fuel in the tank—this is to provide a baseline for the test since gasoline and diesel are volatile fluids that expand and vaporize easily with warm temperatures.
2. When the leak test initiates, the Vapor Canister Vent Valve is closed to prevent any fresh air from entering the EVAP system.
3. The Purge Valve is opened, which allows the engine to create a vacuum in the EVAP system.
4. After a specified time interval—usually about ten seconds—the Purge Valve is shut off and the vacuum level in the system is measured by the Fuel Tank Pressure Sensor.
5. Finally, a countdown initiates, which measures the rate at which the vacuum decays in the system. If the vacuum decays much faster than the specified rate or if no amount of vacuum is reached on two consecutive tests, then the Powertrain Computer will fail the EVAP system for a gross leak and trigger the P0455 code.

How do you determine the cause of a P0455 code?

• The P0455 code is somewhat misleading because the problem may not be a large/gross leak at all. Many systems trigger this code if there is no EVAP flow detected, which is tracked by changes in Short Term Fuel Trim and Fuel Tank Pressure Sensor data. For example, if the Purge Valve is shorted and never closes, it can trigger a P0455. Be ready to think outside of the box when tracking down the cause of a P0455.

• 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.