General Information

Component Location





General Description

The downstream heated oxygen sensor is mounted downstream from the Catalytic Converter (worm-up catalytic converter) or in the rear exhaust pipe. The sensor is used to monitor the efficiency of the Catalytic Converter. Because the Catalyst uses the oxygen in the exhaust, the amount of oxygen coming from the converter is less than what entered the converter. The downstream HO2S signal would usually flat line around.6 to.7 volts with a catalyst the is working properly.

DTC Description

Checking output signals from HO2S under detecting condition, if an output signal is above 1.08V for more than predeterminate time, ECM sets P0138.

DTC Detecting Condition





Diagnostic Circuit Diagram





Signal Waveform & Data




Fig. 1) Normal waveforms of front HO2S (the upper) and rear HO2S (the lower)
In closed loop operation the ECM monitors the HO2S signal. When the sensor voltage rises about.5V the ECM will command a leaner fuel mixture. As the exhaust from the leaner mixture reaches the HO2S the signal voltage will drop below.5V. At that point, the ECM will command a richer mixture which in turn will cause the sensor voltage to rise.