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EVAP system codes

The evaporative emissions (EVAP) system keeps fuel vapor from escaping the tank. It stores the vapor in a charcoal canister and later draws it into the engine to burn. The car tests that system for leaks, and a failed test turns on the check engine light.

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If the light came on right after a fill-up, check the fuel cap first. Strong fuel smell calls for a prompt inspection.

Simplified EVAP vapor path Fuel vapor flows from the tank to a charcoal canister. When the purge valve opens, the engine draws stored vapor into the intake. The vent valve lets fresh air into the canister and closes during the leak test. FUEL TANK FUEL CAP CANISTER PURGE VALVE ENGINE VENT VALVE AIR
Simplified layout. Vapor collects in the canister, the purge valve lets the engine burn it, and the vent valve lets air in. For the leak test, the system is sealed and checked for pressure or vacuum change. Part locations vary by vehicle.

Start at the filler

A loose, damaged or missing fuel cap is the easiest leak to fix, and owner’s manuals from several manufacturers list it first. Tighten the cap until it clicks, then give the car a few drives to rerun its test.

What the leak codes mean

P0455 means a large leak or a system that didn’t seal at all. P0442 means a small leak. P0456 means a very small leak. The size tells the shop how big the leak looked to the test. It doesn’t say where the leak is.

Why the test only runs sometimes

The EVAP monitor runs only when its conditions are met, such as fuel level and temperature, and those conditions vary by vehicle. The light can come on some time after the event that caused the leak, and it can take a few drives to go off after a repair.

This matters for inspections. California’s Smog Check, for example, lets a 2000 or newer gasoline car pass with only the EVAP monitor incomplete. Other states set their own rules.

How a shop finds the leak

A smoke machine made for EVAP systems pushes visible vapor into the system so the technician can see where it escapes. The shop may also test whether the purge and vent valves open and close when commanded. On Nissans covered by bulletin NTB17-082E, the technician checks the seal where the vent valve joins the canister after ruling out the fuel cap.

What else can set an EVAP code

Flow and circuit faults are different from leaks. A purge valve that sticks open or a vent valve with an electrical fault can set its own codes. Copy the exact code, because the test for a valve circuit is different from the test for a leak.

Codes in this guide

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