If the O2 sensor reads above 550 millivolts consistently, what does it indicate about the fuel mixture?

When the O2 sensor consistently reads above 550 millivolts, it indicates that the fuel mixture is likely rich. In the context of oxygen sensors, a higher voltage output, especially above the 450 millivolt threshold, typically signifies that there is an excess of fuel in relation to the amount of oxygen present in the exhaust gases. This translates to more fuel than necessary being supplied for combustion.

The rich condition occurs when the air-fuel mixture has a higher ratio of fuel compared to air, which can lead to incomplete combustion and increased emissions. In contrast, a lean mixture would produce lower voltage readings from the sensor, as there would be more oxygen available than fuel. Normal operation tends to hover around the 450 millivolt range, where the vehicle's engine management system is balanced. An exhaust leak would cause irregular readings or fluctuations rather than consistently high readings, making it an unlikely explanation for a stable high voltage scenario.