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 VCM 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 during 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 an HO2S response switching,
transition time, or ratio problem is detected, the VCM will store a DTC which
indicates degraded an HO2S performance.
In order to control the 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 by using the
Bank 1 Sensor 2 and the Bank 1 Sensor 3 heated oxygen sensors. The Bank 1
Sensor 2 sensor produces an output signal which indicates the amount of oxygen
present in the exhaust gas entering the three-way catalytic converter.
The Bank 1 Sensor 3 sensor produces an output signal which indicates the oxygen
storage capacity of the catalyst; this in turn indicates the ability of the
catalyst to convert the exhaust gases efficiently. If the catalyst is operating
efficiently, the Bank 1 Sensor 2 sensor will produce a far more active signal
than that produced by the Bank 1 Sensor 3 sensor.
The HO2S sensors which are used in order to monitor the catalyst function
are the same as the HO2S sensors which are used in order to control the fuel.
Bank 1 Sensor 2 and Bank 1 Sensor 3 are used primarily for catalyst monitoring.
These sensors 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 the
fuel trim in order to ensure that the fuel delivery is correct for catalyst
monitoring.
