Why does a loose gas cap cause a check engine light?

How does a loose gas cap cause a check engine light?

Vehicles are equipped with an evaporative emissions system that captures fuel vapors as you fill your tank. Venting fuel vapors into the air causes air pollution and smog.

How does the evaporative emissions system work?

As you fill your tank with fuel, vapor pressure builds up in the tank, pushing the fuel vapors into a canister filled with activated charcoal. The charcoal absorbs the fuel vapor and stores it temporarily.

After filling you install the gas cap and start the engine. The vehicle computer knows you’ve filled up the tank because it sees movement with the fuel gauge. It then performs a purge cycle.

How the purge cycle works

With the engine running after a fillup, the computer commands the purge valve and vent valves to open. Engine vacuum sucks the fuel vapor out of the charcoal canister and burns the vapor in the engine. At the same time, the vent valve allows fresh outside air into the charcoal canister to replace the fuel vapors.

The vehicle computer monitors engine RPM and the oxygen sensors and uses that data to determine when the canister is fully purged. It knows this because the engine RPMS drop due to no more fuel vapor coming into the engine from the canister. It then closes the vent valve but leaves the purge valve open. This is the gas cap and fuel system integrity test.

A loose gas cap causes a check engine light because it causes the fuel system integrity test to fail

With the engine running and the purge valve open, engine vacuum puts the entire engine fuel system under a vacuum condition. It then closes the purge valve and monitors the vacuum level in the fuel system using the fuel tank pressure sensor.

A loose gas cap causes a large leak and the computer detects that by how quickly the vacuum falls. The rapid drop causes the computer to set a check engine light and set a code for a large leak. That’s how a loose gas cap causes a check engine light.

What causes a gas cap to leak if it’s actually tight?

Gas caps seal with a rubber gasket against the filler neck. If the filler neck develops rust, the rust can wear out the rubber gasket. Some gas caps fail on their own due to a poorly designed rubber gasket or a failure in the pressure relief valve built into the cap.

A worn gas cap seal can cause a check engine light.

Many Honda gas caps fail due to worn O-ring seal. You can buy replacement seals online and just install the new O-ring on your old cap.

Honda gas cap replacement seal

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