What Is Closed Loop?
Closed loop is the engine management mode in which the computer uses oxygen sensor feedback to continuously correct fuel delivery. It's the normal operating state of a fully warmed engine.
In plain English
In closed loop, the computer meters fuel, watches the oxygen sensors respond, and trims its next decisions — a continuous feedback cycle. This is how modern engines hold a precise mixture for economy and emissions.
The phrase matters diagnostically: many self-tests only run in closed loop, and a vehicle that won't enter closed loop has a problem (temperature, sensor, or heater circuit) that must be fixed first.
Example
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