P0456 Very Small EVAP Leak Code Explained

P0456 means the computer detected a very small leak in the evaporative emissions system, about the size of a 0.020 inch hole. Its close relative P0442 is the small leak code, and on Ford's test that one means a 0.040 inch hole. This page explains how the two codes differ and what manufacturers say causes them.

By Enrique Reyes, who works at a family run auto repair and smog check shop in Delano, California. The shop has been open since 1972. Author Source

The Meaning

What P0456 Means

The federal regulation names P0456 as a generic very small leak code for the 0.020 inch test. Ford's engine off test also sets P0456 for a 0.020 inch leak.

The name on your scan tool may not match, because a 2013 Volkswagen bulletin calls P0456 a small leak, the same words it uses for P0442, and FCA's 2015 bulletin does the same. The federal regulation, a 2019 Volkswagen bulletin and Mazda all call it a very small leak. The code number is what matters.

Side By Side

How P0456 Differs From P0442

Size is the entire difference. P0442 is tied to a 0.040 inch hole, and P0456 is tied to a 0.020 inch hole, which is half as wide.

Ford runs two separate tests. The engine running test seals the system with the vent valve, pulls vacuum with the purge valve and then measures how quickly the vacuum bleeds back, and that test sets P0442. The engine off test runs after the key is turned off, and it checks only for 0.020 inch leaks. It leaves 0.040 inch and larger leaks to the engine running test.

Ford's 2009 summary gives two different rest times for the engine running test. Its text describes a 6 hour cold soak, while its table lists a 3.5 hour minimum. Both figures are in the document.

How The Leak Limit Got Smaller

• MOTOR Magazine, a trade magazine, says the allowed leak dropped from 0.040 inch to 0.020 inch starting in model year 2000, phased in over several years.
• Ford's 2009 document says federal vehicles could use a 0.040 inch test while California vehicles needed the 0.020 inch test.
• The federal OBD regulation says model year 2017 and later vehicles must detect a 0.020 inch leak, although tanks over 25 gallons may get approval for up to 0.040 inch.
• California's current regulation covers both sizes, and manufacturers may ask to use 0.090 inch in place of 0.040 inch.

Causes

Causes Manufacturers Name

No source behind this guide ranks causes by how often they happen, so the lists below simply come from manufacturer bulletins, and each bulletin covers particular models.

Named For P0456

• A gas cap gasket with too much friction, so the cap does not seal, on several 2011 to 2013 Acura models.
• A filler neck leak at the main flap on the 2016 Honda Pilot and 2017 Ridgeline.
• A split purge valve hose on some 2015 Grand Cherokee and Durango models.
• Salt deposits inside the leak detection sensor on 2016 to 2019 Hyundai Sonata Hybrid vehicles.
• Air leaks in the EVAP system or in the leak detection pump on the 2013 to 2014 Audi Q7.
• No natural vacuum detected after shut off on 2012 to 2014 Audi A4, A5, A6 and A7 vehicles.

Named For P0442

• A purge valve stuck by oil from the cam cover gasket on the 2014 to 2018 Impala 3.6L.
• Water, dirt or corrosion at the canister vent valve on 2007 to 2010 General Motors full size SUVs.
• A loose fuel filler cap on some Mazda3, Mazdaspeed3 and Mazda5 models, which Mazda fixed with an improved cap.

Timing

Why The Code Takes Days To Set Or Clear

The test for a very small leak takes time, since Ford's engine off test runs up to twice a day, and it can take up to 45 minutes when a leak is present. For the 0.020 inch test, California lets manufacturers ask to use statistical logic that takes up to 12 drive cycles on average.

Other manufacturers use different methods. Toyota's key off system pulls vacuum through a 0.020 inch reference opening and then compares that reading to the whole system, according to MOTOR Magazine. Audi's natural vacuum system treats 2.5 mbar of vacuum after shut off as sealed.

After a repair, the warning light goes off after three drive cycles in a row where the test runs and passes, and the stored code is erased after 40 warm up cycles.

Next Steps

What To Do About P0456

Start with the gas cap, since a cap that does not seal is on the list above. The Gas Cap page in the menu explains the inspection, and then the test needs time to run again.

If the warning light returns, the leak has to be located. YourMechanic lists a check engine light diagnostic for code P0442 at $169.99 for Ford and Toyota and $184.99 for BMW, including a $50 credit toward the repair. The page shows no price date and was read on November 12, 2026, and it is not a smoke test price.

For bulletins by manufacturer and the repair price rows, evapleak.com keeps a full P0456 code page.

Stuck On A P0456 Code?

Call (866) 389-3808. Have the year, make and model of the vehicle ready.