What Is Engine Code P0456?

  • P0456 Technical Definition: Evaporative Emission System - Small Leak Detected
  • P0456 Meaning: A possible fuel vapor leak 
  • Most common cause: Faulty fuel cap
  • Risks for the engine/driver: LOW You should get it checked out ASAP but your car should be safe to drive
  • Emissions severity: HIGH. The car probably won’t pass emissions testing
  • Estimated repair time: 1 Day
  • Estimated repair cost: $100+

The engine code P0456 is stored in memory when the ECU detects a leak in the Evaporative Emission System (EVAP).

The EVAP is an integral part of the emissions control system, its role is preventing fuel fumes from reaching the atmosphere. These gases are the normal result of fuel evaporating inside the tank. The EVAP system consists of a sealed container (fuel tank) that works in conjunction with one or more charcoal canisters that purge the gases back to the engine when the ECU consider it appropriate.

Besides controlling the EVAP solenoids the Electronic Control Unit has a dedicated fuel tank pressure sensor (FTP sensor). This sensor is constantly informing the ECU about the fuel vapor pressure in the tank but also is a key component during the occasional leak tests performed by the Electronic Control Unit. 

The code P0456 is set when the ECU detects a small leak in the EVAP system during these tests.


2 Common Symptoms When Code P0456 is Present

  1. Check Engine Light 
  2. Gasoline smell coming from the vehicle

Possible Causes For Error Code P0456

The most common causes of data trouble code P0456 are:

  • Aftermarket gas cap that doesn’t meet OEM specifications.
  • Improperly closed gasoline cap.
  • Faulty gas cap.
  • Burnt, damaged, broken or deteriorated EVAP hose.
  • Defective fuel tank seal.
  • Defective EVAP canister.

How To Diagnose The Engine Code P0456

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Preliminary steps

To discard a possible intermittent DTC condition, you’ll need to clear the ECU memory and complete a driving cycle.

  • Read data trouble codes and take note of them.
  • Clear data trouble codes memory.
  • Perform a driving cycle (at least 5-10 minutes).

In case the Check Engine Light stays off then you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.

2. Visual Inspection

> Gas Cap inspection: perform a meticulous visual inspection of the gasoline cap. Check its seal condition. Change as needed. Double check the part number and pressure specifications. Modern gas caps have similar specifications than radiator caps, using the incorrect part can bring the same issue.

> EVAP hoses: perform a meticulous visual inspection of all Evaporative Emission system hoses. These hoses come from the fuel tank all the way through the EVAP canisters and then from the canister they go to the intake manifold. Check for loose clamps, deteriorated hoses, etc. Don’t overlook this step. Many times the code P0456 is caused by a large leak in one of these hoses.

Fix any wiring problem before continuing.

3. Scan Tool Tests

> EVAP functional test: start the engine to enter KOER mode. Depending on the year and maker of your vehicle you may have access to EVAP functional tests. The functional test consists in opening/closing the EVAP purge. If the solenoids are not working then you should check the wiring, connector, relay, and fuse.


How To Repair Error Code P0456

Depending on the diagnostics results you may need to do the following:

  • Replacing the fuel cap.
  • Replacing the fuel tank seal.
  • Replacing or repairing the EVAP hoses.
  • Replacing or repairing the purge solenoids, relay or fuse.

Even when this code doesn’t affect engine’s performance you need to be aware of legal issues when driving in zones with strict emissions regulations. Fuel vapor smell will instantly make you fail emissions regulations.