Rick's Free Auto Repair Advice

Oxygen Sensor Monitor — I/M Oxygen Sensor Monitor

Oxygen Sensor Monitor

In simple terms, the Oxygen Sensor’s only job is to monitor how much oxygen is in the vehicle’s exhaust stream. The ECM uses this information to determine how good of a job it did in calculating the air/fuel ratio based on the current driving conditions (outside air temperature, engine temperature, accelerator pedal position, air mass, etc.). Based on the results, the ECM makes changes to future air/fuel calculations.

How oxygen sensors work

Carmakers use various types of oxygen sensors, with the most recent designs called air/fuel ratio sensors.

Older oxygen sensors generate their own power based on the difference in oxygen content in the exhaust stream compared to oxygen content in the ambient air. If the exhaust gas has a large amount of oxygen (a lean air/fuel mixture), the oxygen sensor generates a “low” voltage signal. If the exhaust gas has very little oxygen (a rich mixture condition), the oxygen sensor generates a “high” voltage signal. In these older sensors, a 450mV signal indicates the most efficient, and least
polluting, air/fuel ratio of 14.7 parts of air to one part of fuel.

Newer air/fuel ratio sensors are far more efficient, respond faster to changes in oxygen concentration and require power to operate. They’re also more expensive.

How the oxygen monitor works

In a normal engine, oxygen concentration varies wildly. So the ECM expects to see swings in oxygen sensor data (see image below). If the ECM see a low or high voltage for long periods, it will set a trouble code and turn on the check engine light.

oxygen sensor scope pattern

Typical oxygen sensor scope pattern

All oxygen sensors require a heater to operate properly

The oxygen sensor must reach a temperature of at least 600-650°F to read properly. To maintain proper minimum temperature, modern oxygen sensor incorporate a heater to keep the sensor at that temperature. That’s why late model vehicles incorporate a separate oxygen sensor heater monitor.

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Posted on by Rick Muscoplat

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