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DESCRIPTION
The Engine Control Module (ECM) uses oxygen sensor signals to maintain the air fuel mixture at the ratio resulting in optimum fuel economy and reduced emissions. The amount of oxygen in the exhaust gases indicates, to the front oxygen sensor, whether the air fuel mixture being supplied to the engine cylinders is rich or lean.

The readings of the rear oxygen sensor are used to indicate the efficiency of the catalytic converter. The ECM calculates catalytic convener efficiency by comparing the rear oxygen sensor signal to the front oxygen sensor signal. A normal oxygen sensor signal will constantly fluctuate above and below 500 mV, with the front oxygen sensor signal frequency of at least 5 Hz at 2500 rpm.

Due to the effect of the catalytic converter, the rear oxygen sensor signal frequency will be lower than the front oxygen sensor signal frequency. If the rear oxygen sensor signal coincides with the front oxygen sensor signal a large percentage of the time, this indicates a loss in efficiency of the catalytic converter or a malfunction within the fuel system.





FAILURE CONDITIONS
After the Front Oxygen Sensor Heater is on for 3 minutes and 20 seconds, the ECM continuously measures the front oxygen sensor's output in 0.5 second intervals. If during two driving cycles the front oxygen sensor's output falls below -128 millivolts for 0.5 seconds, the ECM will set P0131 and MIL will turn on. This code indicates an unusually lean fuel air mixture being read by the front oxygen sensor or ECM.

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TEST PROCEDURE