The Heated Oxygen Sensors are mounted in the exhaust system where they
can monitor the oxygen content of the exhaust gas stream. The oxygen present
in the exhaust gas reacts with the sensor to produce a voltage output. This
voltage should constantly fluctuate from approximately 100 mV (high
oxygen content - lean mixture) to 900 mV (low oxygen content -
rich mixture). The heated oxygen sensor voltage can be monitored with a scan
tool. By monitoring the voltage output of the oxygen sensor, the PCM calculates
what fuel mixture command to give to the injectors (lean mixture - low
HO2S voltage = rich command, rich mixture - high HO2S voltage = lean
command).
When the VCM detects an HO2S signal circuit that is low, the VCM will
set the following DTCs:
When the VCM detects an HO2S signal circuit that is high, the VCM will
set the following DTCs:
A fault in the heated oxygen sensor heater element or its ignition feed
or ground will result in an increase in time to Closed Loop fuel control.
This may cause increased emissions, especially at start-up. When the VCM detects
a malfunction in the HO2S heater circuits, the following DTCs will set:
The VCM also has the ability to detect HO2S response, switching, transition
time, and incorrect ratio voltage problems. If a HO2S response switching,
transition time, or ratio problem is detected, the VCM will store a DTC that
indicates degraded HO2S performance.
To control emissions of Hydrocarbons (HC), Carbon Monoxide (CO), and
Oxides of Nitrogen (NOx), a three-way catalytic converter is used.
The catalyst within the converter promotes a chemical reaction which oxidizes
the HC and CO present in the exhaust gas, converting them into harmless water
vapor and carbon dioxide. The catalyst also reduces NOx, converting it to
nitrogen. The VCM has the ability to monitor this process using the Bank 1
Sensor 2 and the Bank 2 Sensor 2 heated oxygen sensors. The front HO2S sensors
(Bank 1 Sensor 1 and Bank 2 Sensor 1) produces an output signal which indicates
the amount of oxygen present in the exhaust gas entering the three-way catalytic
converter. The rear HO2S sensors (Bank 1 Sensor 2 and Bank 2 Sensor 2) produces
an output signal which indicates the oxygen storage capacity of the catalyst;
this in turn indicates the catalysts ability to convert exhaust gases efficiently.
If the catalyst is operating efficiently, the front sensors will produce a
far more active signal than that produced by the rear sensors.
The catalyst monitor sensors operate the same as the fuel control sensors.
Although the Bank 1 Sensor 2 and Bank 2 Sensor 2 sensors main function is
catalyst monitoring, they also play a limited role in fuel control. If a sensor
output indicates a voltage either above or below the 450 millivolt
bias voltage for an extended period of time, the VCM will make a slight adjustment
to fuel trim to ensure that fuel delivery is correct for catalyst monitoring.
