ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Easy
A technician is diagnosing a vehicle with a strong fuel smell and black smoke from the exhaust. A scan tool shows that the engine's oxygen sensor (O2S) for Bank 1 Sensor 1 (upstream) is consistently reading a high voltage (0.8-0.9V). What does this O2 sensor reading indicate about the air-fuel mixture?
- AA rich air-fuel mixture.
- BAn optimal stoichiometric air-fuel mixture.
- CA fluctuating air-fuel mixture.
- DA lean air-fuel mixture.
Show answer & explanationAnswer & explanation
Correct answer: A. A rich air-fuel mixture.
A consistently high voltage (0.8-0.9V) from a zirconium dioxide (ZrO2) oxygen sensor indicates a rich air-fuel mixture, meaning there is insufficient oxygen in the exhaust. This aligns with the symptoms of a strong fuel smell and black smoke.
Why the other options are wrong
- B. An optimal stoichiometric mixture would cause the O2 sensor voltage to switch rapidly between high and low (around 0.45V average).
- C. A fluctuating mixture is normal for a healthy sensor, but 'consistently high' indicates a steady rich condition, not fluctuation.
- D. A lean mixture would cause the O2 sensor to read low voltage (0.1-0.2V).
Oxygen Sensor Voltage (ZrO2)
A zirconium dioxide (ZrO2) oxygen sensor generates voltage based on the oxygen content in the exhaust, indicating the air-fuel ratio.
- Low voltage (0.1-0.2V) = lean mixture (high O2).
- High voltage (0.8-0.9V) = rich mixture (low O2).
- Rapid switching (0.1-0.9V) = optimal mixture.
Memory trick: O2 sensor: 'High' volts, 'Rich' fuel. 'Low' volts, 'Lean' fuel.