Express 3/4 Ton Van V8-5.7L VIN R (1998)
Heated Oxygen Sensor: Description and Operation
Catalyst Monitor HO2S 2
DESCRIPTION
In order 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.
OPERATION
The VCM has the capability to monitor this process using HO2S 2. HO2S 2, located in the exhaust stream past the three-way catalytic converter,
produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust
emissions effectively. The catalyst monitor sensors operate the same as the fuel control sensors. Although the Bank 1 Sensor 2 and Bank 1 Sensor 3
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.
DIAGNOSIS
A problem with the HO2S 2 electrical circuits should set DTC P0137, P0138 or P0140, depending on the specific condition. If the catalyst is
functioning correctly, the HO2S 2 signal will be far less active than that produced by HO2S 1. If a problem exists which causes the VCM to detect
excessive HO2S 2 activity outside of an acceptable range for an extended period of time, the VCM will set DTC P0420, indicating that the three-way
catalytic converter's oxygen storage capacity is below a threshold considered acceptable.
