What Does the P0456 Code Mean?
To begin with, the P0456 code doesn’t appear on all vehicle platforms, so, depending on the vehicle you drive, you may never see this code at all. Some late model GM vehicles don’t include this code in their list.
Diagnostic trouble code (DTC) P0456 stands for “Evaporative Emission System (EVAP) Small Leak Detected.” Your car’s computer will set this code when it determines that there’s a minor leak somewhere in the evaporative emissions control (EVAP) system.
Typically, the EVAP leak test won’t run unless the fuel level is between 15 and 85 percent. For that reason, people who keep their tanks topped off or always run it very low on fuel may almost never get an EVAP DTC even if there’s a leak. And as far as driving the vehicle with a small leak, you probably won’t notice anything but a Check Engine (MIL) light.
Basically, the evaporative emission control system (EVAP) is a collection of components that work together to prevent fuel vapors from escaping into the atmosphere. This system must be free of leaks to comply with emissions regulations.
To check for leaks, your car’s primary computer, which is often referred to as the powertrain control module (PCM), will perform system self-tests when certain criteria are met.
During a typical “small leak” test, the PCM first opens the purge valve to create a vacuum in the system. The device then seals the EVAP system by closing both the purge valve and the vent valve. For that reason, a slightly leaking canister purge valve or vent valve (internal to the valve can be the source of the leak.
Finally, to check for leaks, the PCM monitors vacuum decay via a fuel tank pressure (FTP) sensor. If a small leak is detected, P0456 or another small leak code will be set.
Typically, the PCM will only set the code if it sees the system fail the self-test twice.
The above description is how most systems operate. Keep in mind, however, that some vehicles use a slightly different method for identifying leaks. For example, some applications may use a leak detection pump and switch.
Older Chrysler and Dodges and some Toyotas, for example, have a device called a “Leak Detection Pump” that is designed to check for EVAP leaks while the vehicle is parked and the engine isn’t running. Later model Chrysler/Dodge vehicles use what they call an “Evaporative System Integrity Monitor,” (ESIM) which is a special switch that takes advantage of the fact that in a sealed fuel tank, pressure rises and falls with changes in temperature.
The basic strategy used in late model Dodge/Chryslers equipped with the ESIM is that in a sealed system, pressure will naturally increase or decrease in relation to temperature. As temperature increases, so does pressure inside the system. Conversely, with a decrease in temperature, vacuum is created if there are no leaks. During cooling, the internal weights seal the evaporative system from the atmosphere and allow the switch to close. If the switch closes it indicates that the system is not leaking.
All vehicles purge stored fuel vapor from the charcoal canister by way of a purge solenoid that allows engine vacuum to draw outside air through the canister. The normally open “Vent Solenoid Valve” allows free passage of air unless it is energized. On Fords, the Vent Solenoid is only closed when the system is testing for an EVAP leak.
What are the Possible Causes of the P0456 Code?
There are several issues that could cause a P0456 code:
- Gas cap is damaged or not closed properly
- Defective purge volume control valve or canister vent control valve
- Faulty EVAP hose
- Leaking charcoal canister
- Damaged fuel tank
- Faulty leak detection pump
- Faulty FTP sensor
What are the Common Symptoms of the P0456 Code?
A vehicle will continue to run even with a P0456 code because EVAP system leaks are often barely noticeable. The only symptoms you may observe that could be associated with this trouble code are the following:
- Illuminated check engine light
- Slightly increased vehicle HC emissions from the leak point
The problem is that if you ignore a check engine light and just keep driving the car, other problems may pile up behind the first one until you wind up with a very large repair bill, so get the first problem fixed rather than ignoring the light.
How to Diagnose the P0456 Code
To diagnose a P0456 code, you need to first make sure that the gas cap is securely fastened. A loose or faulty gas cap can easily trigger the trouble code. If tightening the gas cap doesn’t work, you may want to try purchasing and installing a new cap.
Keep in mind: once you’ve tightened or replaced the gas cap, you’ll have to clear the code with a code reader or scan tool afterward. The code won’t immediately go away on its own.
To be honest, you might have to hire a professional to find a very small leak. Even with the help of a smoke machine and other equipment, small leaks can be extremely difficult to locate. You can use a bright flashlight and do a visual inspection for split hoses or cracked plastic parts. The canister itself can leak as well.
Professionals will use very expensive scan tools to run an Evaporative Monitor procedure to determine whether a leak is present, both before and after repairs. But again, finding a small leak can be extremely difficult.
If a fuel pump has been replaced (gas tank removal), a small leak may result because of the fuel pump seal being out of place or because of some damaged or disconnected components on the tank.
If the gas cap doesn’t solve the problem, you’ll need to dig further. There are numerous potential causes for OBD-II code P0456. As such, diagnosis can be difficult. For an idea of how to troubleshoot the code, check out the videos below:
How to Fix the P0456 Code
There are multiple reasons why code P0456 might be stored. Therefore, there isn’t a “magic bullet” fix for the issue. You’ll need to diagnose the code accurately, as outlined above, then perform any necessary repairs.
Also, keep in mind that all vehicles are different. When troubleshooting and repairing diagnostic trouble codes, you should consult the factory repair information for your application.
Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription might be better. ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.
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Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.