Caprice V8-265 4.3L VIN W MFI (1995)
at the surface and this helps keep the element up to the desired temperature. The HO2S has an inter cavity which is filled with atmospheric (reference)
air. The reference air has approximately 21% oxygen in it. In this electrical circuit this inter cavity is the positive (+) terminal. The outer surface of the
element is exposed to the exhaust gas stream. It is the negative (-) or ground terminal. The oxygen concentration differences between the reference air
and exhaust gases produce small voltages.
A rich exhaust (excessive fuel) has almost no oxygen. When there is a large difference in the amount of oxygen touching the inside and outside surfaces,
there is more conduction, and the sensor puts out a voltage signal above 600 mV. With lean exhaust (excessive oxygen) there is about two percent
oxygen in the exhaust. This is a smaller difference in oxygen from the outside surfaces which results in less conduction and a voltage signal below 300
mV. The voltages are monitored and used by the PCM to "fine tune" the air/fuel ratio to achieve the ideal mixture desired.
When the engine is running lean. the voltage drops below the reference voltage due to excess oxygen in the exhaust stream. The HO2S provides the
feedback information for the "Closed Loop" operating mode of the fuel delivery system. The HO2S indicates to the PCM what is happening in the
exhaust. It does not cause things to happen. It is a type of gage: Low voltage output = lean mixture = high oxygen content in exhaust; high voltage output
= rich mixture = low oxygen content in the exhaust.
An open Heated Oxygen Sensor (HO2S) circuit, should set Diagnostic Trouble Code (DTC) 13 or 63. A constant low voltage in the HO2S circuit could
set a DTC 44 or 64. A constant high voltage in the circuit should set a DTC 45 or 65. DTCs 44. 45. 64, or 65 could also be set as a result of fuel system
problems.
