P0140

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

Test Procedure Part 3:

Circuit Diagram:

Test Procedure Part 1:

Test Procedure Part 2: