Glossary
oxygen sensor
An oxygen sensor sits in the exhaust and measures how much oxygen is left over after the burn, so the computer can tell whether the last mixture ran rich or lean and correct the next one.
An oxygen sensor is the computer's feedback on the burn. It sits in the exhaust, downstream of the combustion chamber, and it reports how much oxygen is left over in the gases leaving the engine. A rich mixture — more fuel than the air could burn — leaves little oxygen behind; a lean one leaves more. Read against the engine's design, that leftover oxygen tells the computer whether the last mixture burned rich or lean, and the computer trims the next one to correct it.
Place it by where it sits and the subtest falls into line. The oxygen sensor is the only sensor here that measures after combustion: the exhaust is the report card, and the computer corrects the mixture from it. The mass airflow sensor works the other end, measuring the air on its way in.
Two failures matter. A dead sensor leaves the computer with no feedback, so the mixture drifts and the catalytic converter cannot clean what it was never sent; the check-engine light comes on and the car fails an emissions test. And an unheated sensor reads nothing until it has warmed, which is why a cold start is held to a looser standard.
Taught in
Related terms
- mass airflow sensor — A mass airflow sensor sits in the intake and measures how much air is on its way into the engine, so the computer can add the matching amount of fuel.
- catalytic converter — A catalytic converter is the exhaust part that cleans the exhaust gases. Hot gases pass over a catalyst coated honeycomb that turns carbon monoxide and other pollutants into less harmful ones.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)