P0456 Code: EVAP Leak Detected - Causes & How to Fix

Gasoline evaporates faster than most other liquids like water because the hydrocarbon molecules in gasoline are mostly non-polar. Gasoline is made of many different molecules, and the majority of them repel each other. Because of this weak molecular attraction, the liquid doesn't stick to itself very well and dissipates into the air very quickly. A cup of gasoline will evaporate much faster than a cup of water. Any exposure to air, such as when pumping gas, will release the fumes causing a strong odor.

This high evaporation rate poses a problem to storing gasoline in vehicle fuel tanks. When the vehicle gets hot, fuel in the tank will try to dissipate more and pressure will build up in the tank. Vehicles are not designed to drive around with sealed, high pressure fuel tanks. Think about opening the gas cap - you would have a small explosion when the gases release, and fuel and gasoline fumes would go everywhere. Therefore, gas tanks have to be vented, so that when pressure rises, it can escape without deforming the fuel tank or spewing gas everywhere.

The EPA other government bodies decided that these evaporated fumes from gas tanks from all the cars on the road should not be released into the environment. That's when the EVAP (Evaporative Emission System) system was created. Instead of venting the gasoline vapors into the environment as pressure rises, it routes them through a series of tubes and into charcoal canisters, usually stored on the front of the vehicle. Once there gets to be so much pent up gasoline fumes in this system, the engine routes it into the intake and engine to burn it and get rid of it.

The EVAP system is a great idea, until things go wrong.

When a leak develops in any of the hoses, charcoal canisters, or gas cap, the car will sense that the pressure in this system has dropped, and it will throw the code. In many cases, not screwing the gas cap back on tight enough is enough to trigger the P0456. In other cases, a mechanic will have to inspect the hoses and tubes to find the leak.

Charcoal canisters store the excess fuel vapor in the emissions control system, and a purge value on the canister opens and closes to dispose of the excess gas into the intake manifold. Sometimes that value gets dirt in it or becomes clogged and stays open all the time, causing the P0456. On some cars, this purge value is electronically operated, and the computer opens it and closes it based on sensors in the EVAP. Sometimes the electronics become faulty and it fails to open or close at the expected time. This can be an issue with the electronically operated purge valve itself or one of the sensors the computer uses to turn it on or off.

Yes, it is safe to drive with this code. Engines don't need the EVAP system to operate, and they will run just fine if it's not working. However, you will fail an emissions test if you have this code. It's a good idea to have it looked at by a mechanic, but try making sure you gas cap is fully attached first - sometimes it's that simple a fix.

Faults in modern EVAP system are uncommon, and some cars don't even use this code at all. When vehicles do come into the shop with this code, it's usually the fuel cap, the purge valve on the canister. This system does have a fail safe vent valve that sometimes isn't able to hold in the pressure, causing leaks. Overall this is not a serious issue, and you should continue driving your car.