What does a constant oxygen sensor reading above 550 millivolts indicate?

A constant oxygen sensor reading above 550 millivolts indicates that the fuel mixture is too rich. In gasoline engines, oxygen sensors monitor the level of oxygen in the exhaust gases to assess the air-fuel ratio. When the reading consistently exceeds 550 millivolts, it suggests that there is an excess of fuel in the mix, meaning there is insufficient oxygen for complete combustion. This rich condition can lead to a variety of performance issues, including reduced fuel efficiency, increased emissions, and potential damage to the catalytic converter over time. Monitoring oxygen sensor readings is crucial for engine performance and emissions control, making this indication of a rich fuel mixture an important diagnostic insight.