Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairEasy
A technician is performing an emissions test on a vehicle. The test results show consistently low oxygen (O2) sensor voltage readings (close to 0.1V) at operating temperature and cruising speeds, even though the engine appears to be running smoothly. What does this indicate about the air-fuel mixture?
- AThe air-fuel mixture is stoichometric.
- BThe O2 sensor is stuck in open loop.
- CThe engine is running lean.
- DThe engine is running rich.
Show answer & explanationAnswer & explanation
Correct answer: C. The engine is running lean.
A properly functioning oxygen sensor generates low voltage (around 0.1V) when it detects excess oxygen in the exhaust, which indicates a lean air-fuel mixture. High voltage (around 0.9V) indicates a rich mixture (less oxygen). Consistently low voltage readings point to a lean condition.
Why the other options are wrong
- A. A stoichometric mixture would cause the O2 sensor voltage to switch rapidly between high and low, not stay consistently low.
- B. The O2 sensor provides feedback for closed-loop operation. If the sensor is stuck, it might prevent closed-loop, but the voltage reading itself directly indicates the mixture.
- D. A rich condition would result in high oxygen sensor voltage (close to 0.9V) due to less oxygen in the exhaust.
Oxygen Sensor Voltage (Lean)
A low voltage output (typically 0.1V - 0.2V) from an oxygen sensor indicates a lean air-fuel mixture, meaning there is an excess of oxygen in the exhaust gases.
- O2 sensor measures oxygen content in exhaust.
- Low voltage = high oxygen = lean mixture.
- High voltage = low oxygen = rich mixture.
- Voltage should switch rapidly around 0.45V for stoichiometric.
Memory trick: Low Voltage, Lean Air - Lots of Oxygen There!