Freeze Frame
OBD-II trouble code

P0456

Evaporative Emission System Leak Detected (Very Small Leak)

Severity Moderate Driving See conditions below System Evaporative emissions and fuel-vapor containment Applies to OBD-II gasoline and gasoline-hybrid vehicles equipped with a very-small-leak EVAP monitor

Published · Reviewed and updated

In plain English: The fuel-vapor system failed its very-small-leak check. A loose cap is one possibility, but this code does not identify the leaking part.

If the engine runs normally with no fuel smell or visible fuel, a short trip is usually possible while arranging diagnosis. Stop for liquid fuel, a strong persistent fuel odor or severe running problems. Keep fuel vapors away from flames and sparks.

A diagnostic starting point: Loose cap, damaged cap seal or capless filler seal. Start by asking: Is the cap or capless filler sealing correctly? Illustrative parts budgets span $10–$900 USD, with diagnosis, labor and taxes extra.

Quick answers about P0456

What does P0456 mean?
The fuel-vapor system failed its very-small-leak check. A loose cap is one possibility, but this code does not identify the leaking part.
What should I check first for P0456?
One starting point is Loose cap, damaged cap seal or capless filler seal. An accessible first inspection. Check the correct cap, sealing lip and closure; use the approved procedure for capless hardware. The code alone does not identify a failed part; follow the procedure for your vehicle.
Can I drive with P0456?
If the engine runs normally with no fuel smell or visible fuel, a short trip is usually possible while arranging diagnosis. Stop for liquid fuel, a strong persistent fuel odor or severe running problems. Keep fuel vapors away from flames and sparks.
How much does it cost to fix P0456?
Illustrative parts budgets in this guide span $10–$900 USD. Diagnosis, labor and taxes are extra. Pay for diagnosis that identifies the failed path. Several guessed parts can cost more than one properly performed leak test.
How do I diagnose P0456?
Start with: Is the cap or capless filler sealing correctly? Save all codes and freeze-frame data first. Inspect the cap and neck, then visible hoses and recent repairs. Refit a loose correct cap or repair visible damage. Do not pry capless flaps open or add an improvised plug; follow the owner's manual. Check applicable service bulletins early.

Watch the P0456 videos

Full diagnostic video · 9:46 YouTube Short · 1:05

Full walkthrough — 9:46. Watch on YouTube

What P0456 actually means

Fuel vapor travels from the tank into a charcoal canister instead of escaping outdoors. A purge valve meters stored vapor into the engine. To check for leaks, the vehicle seals the relevant vapor path and evaluates pressure, vacuum or a leak-detection device. Some systems run a pump; others use cooling after shutdown. The monitor judges sealing performance, not the identity of a failed part.

Many systems associate P0456 with a leak equivalent to a hole about twenty thousandths of an inch across. That is a reference size, not a hole the computer measured. Fuel level, temperature, timers and pass limits depend on the vehicle. A monitoring fault can also mimic leakage, so the code alone cannot prove a cap, valve or canister failed.

Fuel tank Intake Charcoal canister — traps the vapour Purge valve Vent valve ← to atmosphere Cap or capless seal One possible leak point, not a verdict
Schematic illustration; the vehicle's layout may differ. A very-small-leak test failed. Locate the fault before replacing parts.

Symptoms

Possible causes and what to check

This is a practical diagnostic order, not measured failure rates. Follow the tests and priorities specified for your vehicle; a code alone does not identify a failed part.

1 Loose cap, damaged cap seal or capless filler seal
An accessible first inspection. Check the correct cap, sealing lip and closure; use the approved procedure for capless hardware.
2 Leaking hose, connector or small seal
A split line or poorly seated joint can leak vapor without dripping fuel. Inspect recent repair areas and the filler connections.
3 Purge valve that leaks when it should seal
An electrical click does not prove sealing. Test the valve's closed state using the vehicle's specified method.
4 Vent valve or leak-module sealing problem
The valve seat, housing or mounting seal can leak. Confirm the exact point before deciding which service part is needed.
5 Canister, filler-neck or tank-top leak
Hidden joints need leak localization. Inspect for a proven leak before paying to remove the tank or replace the canister.
6 Pressure sensing, monitor hardware or calibration fault
If the vapor path passes testing, check sensor response, wiring and applicable manufacturer bulletins instead of continuing to replace seals.

Illustrative vacuum hold: sealing versus leakage

Illustrative graph, not a recording from your vehicle or a universal service limit. Confirm the test conditions, units and acceptance criteria in the applicable service procedure.

The useful comparison is whether a correctly isolated system holds the specified test condition.

This is an illustrative vacuum-hold test, not a universal P0456 specification. Start after test vacuum is established and the required valves are closed. Follow the vehicle's commanded test and monitor conditions; pump and natural-vacuum systems can produce different traces.

-6.0 -4.6 -3.2 -1.8 -0.4 1.0 0 24 48 72 96 120 EVAP pressure relative to atmosphere (inches H2O) Seconds after test vacuum is established Sealed example - slight drift Leaking example - vacuum decays
Holds within the factory limit = sealed during this test. Returns toward atmosphere = locate the loss and verify setup.

The sealed example changes only slightly during the hold. Whether that is a pass depends on the factory limit, fuel level and temperature. A sensor value that looks flat is useful only if the sensor and test setup are working.

The leaking example moves toward atmospheric pressure. This suggests air is entering the isolated path, but warming fuel or an incorrectly closed valve can also change pressure. This graph cannot identify the leaking part or distinguish every leak code by itself.

Check the cap or capless seal first. If the fault remains, use the factory leak test and approved low-pressure EVAP smoke equipment to locate it. Test purge and vent sealing, then verify the repair.

How to diagnose it, in order

  1. Is the cap or capless filler sealing correctly?
    Save all codes and freeze-frame data first. Inspect the cap and neck, then visible hoses and recent repairs. Refit a loose correct cap or repair visible damage. Do not pry capless flaps open or add an improvised plug; follow the owner's manual. Check applicable service bulletins early.
    Yes → go to step 2
    No → Correct the filler fault and repeat the specified leak check
  2. Does the manufacturer-approved small-leak test confirm a leak?
    Meet the required fuel-level and temperature conditions. Verify pressure-sensor plausibility and run the available service-bay test. An incomplete or aborted monitor is not a pass. If the test passes now, examine intermittent conditions and the monitor itself before replacing anything.
    Yes → go to step 3
    No → Investigate intermittent leakage, test setup, sensing and applicable software
  3. Does the purge valve seal in its specified closed state?
    Use the factory procedure to isolate and test this path. A commanded value or audible click proves neither closure nor leak-tightness. Avoid mouth testing a fuel-vapor component. If leakage is proven at the valve, repair it and test the system again.
    Yes → go to step 4
    No → Repair the proven purge leak, then verify the system
  4. Does the vent or leak-detection assembly seal correctly for the test?
    Identify how this design closes its atmospheric path. Check operation and mounting seals, including combined modules. The correct repair might be a serviceable seal or an assembly, depending on the current parts information. Do not assume a click means it holds pressure.
    Yes → go to step 5
    No → Repair the confirmed valve, seal, module or control fault
  5. Does an approved smoke or flow test pinpoint the remaining leak?
    Use purpose-built EVAP equipment, the approved gas and adapter, and the exact low-pressure limit. Keep the engine off and ignition sources away. Never connect oxygen or unregulated shop air to the tank. Inspect hoses, filler seals, canister seams and tank-top joints; verify the tester connection also seals.
    Yes → Repair the located leak and repeat the factory verification test
    No → Recheck isolation and intermittent conditions, then diagnose the monitor

Repeat the factory leak check and confirm monitor completion. Clearing the light does not prove a repair.

What you need to do this yourself

Some links may be affiliate links. They cost you nothing extra and do not change which tools are recommended — the diagnostic order on this site is the same either way.

Illustrative repair budgets

These are rough US planning budgets, not measured repair averages or shop quotes. Parts and labor are separate; diagnosis, taxes and difficult access can add cost.

Illustrative budgets in USD; labor time is shown separately
RepairParts (USD)Labor time
Correct cap or accessible sealing part
Refitting a loose cap may require no replacement part. A capless filler assembly is a different repair.
 $10–$80 0.3 h
Accessible hose or connector repair
A hidden or molded line can cost substantially more to access and replace.
 $10–$150 1.0 h
Purge or vent valve
Price and labor depend on the layout and whether seals are supplied separately.
 $30–$300 1.0 h
Canister or leak-detection module
Replace the assembly only after locating its leak or proving its monitoring fault.
 $100–$700 2.0 h
Filler assembly or tank-top sealing repair
Tank removal, corrosion and combined assemblies can exceed this planning range.
 $50–$900 3.0 h

Pay for diagnosis that identifies the failed path. Several guessed parts can cost more than one properly performed leak test.

P0456 video summary · 1:05

A brief introduction to the code and a diagnostic starting point. Use the full guide and vehicle-specific service information for the complete procedure.

Watch this Short on YouTube

Written diagnostic walkthrough

Current written guidance organized by topic. This text may include corrections made after a video was published and is not a verbatim transcript of the archived video.

Meaning and driving guidance

Code P0456. Evaporative Emission System Leak Detected (Very Small Leak).

In plain English: the fuel-vapor system failed its very-small-leak check. A loose cap is one possibility, but this code does not identify the leaking part.

If the engine runs normally with no fuel smell or visible fuel, a short trip is usually possible while arranging diagnosis. Stop for liquid fuel, a strong persistent fuel odor or severe running problems. Keep fuel vapors away from flames and sparks.

How it is measured

Here's what your engine computer is actually measuring.

Fuel vapor travels from the tank into a charcoal canister instead of escaping outdoors. A purge valve meters stored vapor into the engine. To check for leaks, the vehicle seals the relevant vapor path and evaluates pressure, vacuum or a leak-detection device. Some systems run a pump; others use cooling after shutdown. The monitor judges sealing performance, not the identity of a failed part.

Many systems associate P0456 with a leak equivalent to a hole about twenty thousandths of an inch across. That is a reference size, not a hole the computer measured. Fuel level, temperature, timers and pass limits depend on the vehicle. A monitoring fault can also mimic leakage, so the code alone cannot prove a cap, valve or canister failed.

First establish whether the system actually leaks. Then locate that leak, or diagnose the monitor if sealing tests pass.

Symptoms

What you'll usually notice, beyond the light itself.

Check-engine light with otherwise normal driving. A vapor leak can be too small to affect how the engine feels.

The light appears after refueling or a later parked period. The monitor may need a particular drive followed by an engine-off test, so timing alone does not identify the leak.

An intermittent fuel odor. Odor supports inspecting the fuel system, but a tiny leak may have no detectable smell. Strong odor needs prompt attention.

Repeated failed checks after the code was cleared. Turning off the warning light does not repair the sealing fault or demonstrate that the monitor passed.

Possible causes

Here is a practical order for checking the possible causes. These are diagnostic priorities, not measured failure rates.

Check 1: Loose cap, damaged cap seal or capless filler seal. An accessible first inspection. Check the correct cap, sealing lip and closure; use the approved procedure for capless hardware.

Check 2: Leaking hose, connector or small seal. A split line or poorly seated joint can leak vapor without dripping fuel. Inspect recent repair areas and the filler connections.

Check 3: Purge valve that leaks when it should seal. An electrical click does not prove sealing. Test the valve's closed state using the vehicle's specified method.

Check 4: Vent valve or leak-module sealing problem. The valve seat, housing or mounting seal can leak. Confirm the exact point before deciding which service part is needed.

Check 5: Canister, filler-neck or tank-top leak. Hidden joints need leak localization. Inspect for a proven leak before paying to remove the tank or replace the canister.

Check 6: Pressure sensing, monitor hardware or calibration fault. If the vapor path passes testing, check sensor response, wiring and applicable manufacturer bulletins instead of continuing to replace seals.

Use the test results to choose the repair. A position in this list does not prove that a part has failed.

Illustrative diagnostic pattern

The useful comparison is whether a correctly isolated system holds the specified test condition.

This is an illustrative vacuum-hold test, not a universal P0456 specification. Start after test vacuum is established and the required valves are closed. Follow the vehicle's commanded test and monitor conditions; pump and natural-vacuum systems can produce different traces.

The sealed example changes only slightly during the hold. Whether that is a pass depends on the factory limit, fuel level and temperature. A sensor value that looks flat is useful only if the sensor and test setup are working.

The leaking example moves toward atmospheric pressure. This suggests air is entering the isolated path, but warming fuel or an incorrectly closed valve can also change pressure. This graph cannot identify the leaking part or distinguish every leak code by itself.

Check the cap or capless seal first. If the fault remains, use the factory leak test and approved low-pressure EVAP smoke equipment to locate it. Test purge and vent sealing, then verify the repair.

Diagnostic steps

So here is the order to actually test this, cheapest and most likely first.

Step 1. Is the cap or capless filler sealing correctly? Save all codes and freeze-frame data first. Inspect the cap and neck, then visible hoses and recent repairs. Refit a loose correct cap or repair visible damage. Do not pry capless flaps open or add an improvised plug; follow the owner's manual. Check applicable service bulletins early.

Step 2. Does the manufacturer-approved small-leak test confirm a leak? Meet the required fuel-level and temperature conditions. Verify pressure-sensor plausibility and run the available service-bay test. An incomplete or aborted monitor is not a pass. If the test passes now, examine intermittent conditions and the monitor itself before replacing anything.

Step 3. Does the purge valve seal in its specified closed state? Use the factory procedure to isolate and test this path. A commanded value or audible click proves neither closure nor leak-tightness. Avoid mouth testing a fuel-vapor component. If leakage is proven at the valve, repair it and test the system again.

Step 4. Does the vent or leak-detection assembly seal correctly for the test? Identify how this design closes its atmospheric path. Check operation and mounting seals, including combined modules. The correct repair might be a serviceable seal or an assembly, depending on the current parts information. Do not assume a click means it holds pressure.

Step 5. Does an approved smoke or flow test pinpoint the remaining leak? Use purpose-built EVAP equipment, the approved gas and adapter, and the exact low-pressure limit. Keep the engine off and ignition sources away. Never connect oxygen or unregulated shop air to the tank. Inspect hoses, filler seals, canister seams and tank-top joints; verify the tester connection also seals.

Repeat the factory leak check and confirm monitor completion. Clearing the light does not prove a repair.

Illustrative repair budgets

These are rough US planning budgets, not measured repair averages or shop quotes. Parts and labor are separate; diagnosis, taxes and difficult access can add cost.

Correct cap or accessible sealing part: 10 to 80 dollars in parts, plus about 0.3 hours of labor. Refitting a loose cap may require no replacement part. A capless filler assembly is a different repair.

Accessible hose or connector repair: 10 to 150 dollars in parts, plus about 1.0 hours of labor. A hidden or molded line can cost substantially more to access and replace.

Purge or vent valve: 30 to 300 dollars in parts, plus about 1.0 hours of labor. Price and labor depend on the layout and whether seals are supplied separately.

Canister or leak-detection module: 100 to 700 dollars in parts, plus about 2.0 hours of labor. Replace the assembly only after locating its leak or proving its monitoring fault.

Filler assembly or tank-top sealing repair: 50 to 900 dollars in parts, plus about 3.0 hours of labor. Tank removal, corrosion and combined assemblies can exceed this planning range.

Pay for diagnosis that identifies the failed path. Several guessed parts can cost more than one properly performed leak test.

Sources and application limits

These manufacturer documents and technical references support the discussion. A bulletin may apply only to its listed models, engines and production dates; confirm coverage before applying it.

P0442 P0455 P0446 P0496