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Air Fuel Ratio Sensor

Bulletin 2008

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All engine operating systems requiring this sensor technology


The main function of the on-board engine control module is to maintain and monitor the vehicles emissions to meet the EPA mandated level of the vehicle model year.


In order to monitor the exhaust emission level, the exhaust oxygen level is measured using either an oxygen sensor or an air fuel ratio sensor. The oxygen sensor has been used widely since around the 1980 model year continuing until today. But the capability of the sensor is limited to understanding one thing, an air/fuel ratio of 14.7 to 1. This air fuel ratio has a name, it’s called stoichiometric. The word stoichiometric simply means the substances involved, air and fuel, are at the exact ratio to complete the task at hand. But 14.7 grams of air to 1 gram of fuel are not the perfect mixture under the many operating conditions of the motor and the altitude that the engine is operating in. The computer needs to make decisions based on unknowns in order to keep the engine operating effectively. But effectively is not always efficiently.


This is where the air fuel ratio sensor comes in. When the air fuel ratio sensor is teamed up with a processor programmed with the right software, the ability to monitor air fuel ratios can become as wide or wider than a ratio of 9:1, which is very rich, to 20:1, which is very lean. The response rate of this sensor can be less than 100 milliseconds. For the sensor to function correctly the sensor must be maintained at a higher temperature, approximately 1,200 f. Maintaining this temperature requires a heater element inside the sensor housing which the engines computer system controls and monitors for functionality.


Testing this type of sensor cannot be done with common test equipment, the engine computer must be relied on to report the sensors ability to keep the exhaust emissions within acceptable levels. The only element within the sensor that could be tested would be the heater circuit. Following the trouble code test procedure will aid in testing this circuit. As for a trouble code indicating the air fuel ratio sensor circuit has failed, follow the associated trouble code procedure but trust the computer to provide reliable test results.


For addition support contact the GoTech technical services team at 855-207-5630 M-F 7:30-6:00 CST for assistance.

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