P0455 and your Nevada smog check

Start at the fuel door. Of all the codes that keep a car from passing a smog check, P0455 is the one most often tied to something you can look at yourself: a gas cap that was left loose, cross-threaded or never put back. The Nevada DMV names a loose gas cap as a common cause when the light comes on suddenly. If the cap is fine, the code points to a larger problem a repair shop will need to find, and the smog rules below explain how to time it.

At a glance

• P0455 is the SAE J2012 code for a large leak in the evaporative emission system.
• The DMV names a loose gas cap as a common cause of a check engine light that appears suddenly.
• Tightening the cap does not switch the lamp off at once; the computer needs a few drives to recheck.
• Any lit lamp fails a 1996 or newer light-duty vehicle at the Nevada OBD test.

Gas cap first

Take the cap off and look at it. Is the seal cracked or missing? Was it clicked tight after the last fill-up? Did someone leave it on the pump? A cap that does not seal leaves the fuel tank open to the air, and to the computer that looks exactly like a big hole in the system.

Tighten or replace it, then drive normally. The DMV says tightening the cap may turn the light off after a few days, and EPA's consumer guidance says if the light stays on after a few trips, a technician should look at the vehicle. That delay is the reason to do this well before your registration deadline, not the morning of the test.

What the code actually describes

The generic SAE J2012 wording is: Evaporative emission system leak detected (large leak). Gasoline gives off vapor even when the engine is off. The evaporative emission system, usually shortened to EVAP, collects that vapor in a charcoal canister and later feeds it into the engine to be burned, instead of letting it vent into the air.

To make sure the system is sealed, the computer runs a leak test on its own schedule, often after the car has sat for a while. When the system cannot hold the pressure or vacuum the test expects, and the loss is large, it sets P0455. Smaller losses set P0442 or P0456 instead.

The smog check result with P0455 stored

A leak does not change how the car drives, but it changes the test. Nevada's OBD inspection fails any 1996 or newer light-duty vehicle with the check engine lamp on, whichever system set it.

If the cap fixes it, wait for the light to go out and then come in. If not, the DMV still recommends testing: the failed report is the official record of why the car fails and the first document in the waiver path. One thing to skip is clearing the code with a scanner. The EVAP check is one of the self-tests the computer has to repeat, and a cleared memory brings a not ready rejection.

If the cap was not it

Xpress Smog Check Center tests vehicles; the next step is a repair shop. Common causes a repair shop checks include:

• The filler neck and its seal where the cap closes.
• Cracked or disconnected EVAP hoses.
• The charcoal canister and its connections.
• A vent or purge valve that stays open when it should close.

Emissions coverage and the waiver

EVAP parts are emission control equipment, and the DMV advises checking with the dealer for federal emissions warranty coverage or a recall before paying; federal law covers some expensive emission components for up to 8 years or 80,000 miles. If repairs in Washoe County do not clear the lamp, the waiver rules apply: receipts showing at least $200 at a 2G licensed authorized station, or at least $200 in related parts bought by the owner within 14 days of the initial test, then a visit to the DMV Emission Lab with both failed certificates.