Oxygen Sensor: Description and Operation

Heated Oxygen Sensor (HO2S):

UPSTREAM OXYGEN SENSOR


PURPOSE
The Heated Oxygen Sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected.

OPERATION
A high concentration of oxygen, lean Air / Fuel (AF) ratio in the exhaust produces a low voltage signal less than 0.4 volt. A low concentration of oxygen, "rich AF ratio" produces a high voltage signal greater than .6 volt.

The HO2S provides feedback to the Powertrain Control Module (PCM)indicating AF ratio in order to achieve a near stoichiometric AF ratio of 14.7:1 during closed loop engine operation. This HO2S generates a voltage between O.0 and 1.1 volts.



DOWNSTREAM OXYGEN SENSOR

Heated Oxygen Sensor (HO2S):

PURPOSE
The Heated Oxygen Sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected.

OPERATION

How The Oxygen Sensor Works
A high concentration of oxygen, lean Air / Fuel (AF) ratio in the exhaust produces a low voltage signal less than 0.4 volt. A low concentration of oxygen, "rich AF ratio" produces a high voltage signal greater than .6 volt.


How The Rear HO2S Is Used
The rear HO2S is used during the Federal Test Procedure (FTP) to indicate the efficiency of the Three-way Catalytic (TWC) Converter to store oxygen.

How A TWC Fails
As a catalysts efficiency degrades, its ability to store oxygen declines, thereby decreasing the ability of the TWC to effectively lower Hydrocarbons (HC). This leads to the presence of more oxygen in the exhaust which is "sensed" by the rear oxygen sensor and increases the switching ratio.

Federal Test Procedure
In this procedure, the catalyst efficiency monitor calculates the switching ratio between the front and rear HO2S. This ratio is compared against an emission threshold value. If the switch ratio is above this value, then the TWC is considered to have failed.