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A/F Sensor Monitor
The A/F Sensor monitor is similar to the O2 Sensor monitor; however, the A/F Sensor has different characteristics. Therefore, the operating parameters of the monitor also differ.

The A/F Sensor monitor checks for sensor circuit malfunction, slow response rate, and for a malfunction of the sensor’s heater circuit. There is a DTC for each condition for each sensor. A/F Sensors are required to be monitored once per trip; however, the ECM does continuously monitor A/F sensor operation.

When the ECM sees the right conditions, the ECM will test the A/F sensors for performance by measuring the signal response as the fuel injected into the cylinder is varied. The faster the A/F sensor responds, the better the sensor. The results of this monitor test are NOT reported in Mode 5. Mode 6 is used to determine if the A/F sensor passed or failed. The repair confirmation drive pattern in the Repair Manual provides the driving conditions for the ECM to operate the A/F sensor monitor.



Air/Fuel Ratio Sensor
The A/F sensor is similar to the oxygen sensor. It appears similar to the oxygen sensor, but it is constructed differently and has different operating characteristics.

The A/F sensor is also referred to as a wide range or wide ratio sensor because of its ability to detect air/fuel ratios over a wide range. The advantage of using the A/F sensor is that the ECM can more accurately meter the fuel reducing emissions.

To accomplish this, the A/F sensor: operates at approximately 1200°F (650°C), much hotter than the oxygen sensor 750°F (400°C) changes its current (amperage) output in relation to the amount of oxygen in the exhaust stream



Operation A detection circuit in the ECM detects the change and strength of current flow and puts out a voltage signal relatively proportional to exhaust oxygen content.

NOTE This voltage signal can only be measured by using the Diagnostic Tester or OBD II compatible scan tool. The A/F sensor current output cannot be accurately measured directly. If an OBD II scan tool is used refer to the Repair Manual for conversion, for the output signal is different.
The A/F sensor is designed so that at stoichiometry, there is no current flow and the voltage put out by the detection circuit is 3.3 volts. A rich mixture, which leaves very little oxygen in the exhaust stream, produces a negative current flow. The detection circuit will produce a voltage below 3.3 volts. A lean mixture, which has more oxygen in the exhaust stream, produces a positive current flow. The detection circuit will now produce a voltage signal above 3.3 volts.



The A/F sensor voltage signal is proportional to the change in the air/fuel mixture. This allows the ECM to more accurately judge the exact air/fuel ratio under a wide variety of conditions and quickly adjust the amount of fuel to the stoichiometric point. This type of rapid correction is not possible with the narrow range oxygen sensor. With an A/F sensor, the ECM does not follow a rich lean cycle.