C 3500 Truck 2WD V8-350 5.7L VIN K TBI (1995)
Oxygen Sensor: Description and Operation
Heated Oxygen Sensor (HO2S)
Heated Oxygen Sensor
PURPOSE:
The Heated Oxygen Sensor (HO2S) is essentially a small variable battery; it has the ability to produce a low voltage signal that feeds information
on engine exhaust oxygen content to the control module.
CONSTRUCTION:
The HO2S is constructed from a zirconia/platinum electrolytic element. Zirconia is an electrolyte that conducts electricity under certain chemical
conditions. The element is made of a ceramic material and is an insulator when cold. At operating temperature, 315°C (600°F), the element
becomes a semiconductor. A platinum coating on the outer surface of the element stimulates further combustion of the exhaust gases right at the
surface and this helps keep the element up to the desired temperature. Also a heating element is incorporated into the sensor to aid in warm up to
the operating temperature and also to maintain that temperature. The HO2S has an inner cavity which is supplied with atmospheric (reference) air
from the sensor wires. The atmosphere has approximately 21% oxygen in it. In the electrical circuit this inner 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.
Due to the electrolytic properties of the element the oxygen concentration differences between the reference air and the exhaust gases produce
small voltages.
OPERATION:
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 0.6 volt (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 0.3 volt (300 mV). The
voltages are monitored and used by the control module to "fine tune" the air/fuel ratio to achieve the ideal mixture desired.
The control module puts out a reference signal of 0.45 volt (450 mV). The reference signal serves two purposes. The first is to run the engine
when it is in "Open Loop" mode of operation. When the air/fuel ratio is correct the control module "senses" 450 mV. When the engine is operating
with a rich air/fuel ratio there is a reduction of free oxygen in the exhaust stream and the oxygen sensor voltage rises above the reference voltage.
When the engine is running lean the voltage drops below the reference voltage due to excess oxygen in the exhaust stream. The oxygen sensor
provides the feedback information for the "Closed Loop" operating mode of the fuel delivery system. The oxygen sensor indicates to the control
module what is happening in the exhaust. It does not cause things to happen. It is a type of gauge: Low voltage output = lean mixture = high
oxygen content in exhaust; high voltage output = rich mixture = low oxygen content in the exhaust.
CONDITIONS THAT CAN SET CODES:
An open oxygen sensor, should set a DTC 13. A constant low voltage in the oxygen sensor circuit should set a DTC 44. A constant high voltage in
the circuit should set a DTC 45. DTC 44 and DTC 45 could also be set as a result of fuel system problems. Refer to "DTC Charts" for conditions
that can cause a lean or rich system. The integral heater in the sensor will not set a DTC but is diagnosed in DTC 13 trouble chart and "Diagnostic
Aids."
