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The Engine Control Module (ECM) uses oxygen sensor signals to maintainthe air fuel mixture at the ratio resulting in optimum fuel economy and reducedemissions. The amount of oxygen in the exhaust gases indicates, to the frontoxygen sensor, whether the air fuel mixture being supplied to the engine cylindersis rich or lean. The readings of the rear oxygen sensor are used to indicatethe efficiency of the catalytic converter. The ECM calculates catalytic converterefficiency by comparing the rear oxygen sensor signal to the front oxygensensor signal.

A normal oxygen sensor signal will constantly fluctuate above and below500 mV, with the front oxygen sensor signal frequency of at least 5Hz at 2500RPM. Due to the effect of the catalytic converter, the rear oxygen sensorsignal frequency will be lower than the front oxygen sensor signal frequency.If the rear oxygen sensor signal coincides with the front oxygen sensor signala large percentage of the time, this indicates a loss in efficiency of thecatalytic converter or a malfunction within the fuel system.

[FOR P0130]

After the engine runs, the ECM checks the front oxygen sensor once perdriving cycle for 5 sec. If during two driving cycles the front oxygen sensor'soutput is not between 0 millivolts and +380 millivolts while the rear oxygensensor's outputis greater than 350 millivolts, the ECM will set a code andthe MIL will turn on. This code indicates uncharacteristic operation of thefront oxygen sensor being read by the ECM.

[FOR P0136]

After the system is in closed-loop operation and the engine has runfor 3 minutes, the ECM checks the rear oxygen sensor's output. If during twodriving cycles the rear oxygen sensor's output is not between 400 millivoltsand 500 millivolts, the ECM will set a code and the MIL will turn on. Thiscode indicates an uncharacteristic reading by the rear oxygen sensor or ECMafter the engine has been warmed and run for 3 minutes.