Which statement best describes the status indicated by an O2 sensor switching voltage?

• A. The O2 sensor is switching voltage, indicating closed-loop operation
• B. The Coolant Temperature sensor is faulty
• C. The MAF is out of range
• D. The Engine Control Module is in idle

Answer: (D)

O2 sensor switching voltage shows that the engine is in closed-loop fuel control. When the sensor voltage swings between low and high as the exhaust gas composition changes, the engine computer uses that feedback to adjust the fuel delivery and keep the air-fuel ratio near the stoichiometric target for efficient combustion and catalytic converter operation. This switching behavior is characteristic of normal, active feedback control, which can occur at idle and throughout operation once the system is warmed up.

The other possibilities don’t fit this signal: a coolant temperature sensor fault would affect warm-up behavior and may keep the system in open loop; a MAF out of range would cause readings that don’t reflect proper air flow; the ECM being in idle isn’t determined by the O2 sensor’s switching pattern.