Glossary
charcoal canister
The charcoal canister traps fuel vapor from the tank on activated charcoal instead of letting it escape, and holds it until the engine can purge the canister and burn it.
The charcoal canister is the only emissions part on this subtest that stores anything. Gasoline evaporates — that is what makes it useful and what makes it a problem, because the vapor is both a pollutant and a loss. So the tank is sealed and vented through a canister packed with activated charcoal, whose enormous internal surface holds the vapor instead of releasing it.
The canister is not a disposal site, it is a holding pen. Once the engine is running, the computer opens a purge valve and manifold vacuum draws fresh air through the canister, sweeping the trapped vapor into the intake to be burned. The cycle then repeats: fill with vapor while parked, empty while driving.
It is also why a loose or missing fuel cap can light the check-engine light: the tank is meant to hold a slight pressure, the computer tests for it, and a cap that does not seal reads as a leak. The boundary against the other two emissions parts is clean — the PCV valve handles crankcase vapor, the EGR valve recirculates exhaust gas, and only the canister deals with fuel tank vapor.
Taught in
Related terms
- EGR valve — The EGR valve sends a small, metered amount of exhaust gas back into the intake, which cools the burn and holds down the nitrogen oxides an engine makes when it runs hot.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)