Checking The Oxygen Sensor

PURPOSE:
To obtain a high purification rate for the CO, HC and NOx components of the exhaust gas, a three-way catalytic converter is used, but for the most efficient use of the three-way catalytic converter, the air-fuel ratio must be precisely controlled so that it is always close to the stoichiometric air-fuel ratio.

OPERATION:
The oxygen sensor has the characteristic whereby its output voltage changes suddenly in the vicinity of the stoichiometric air-fuel ratio. This characteristic is used to detect the oxygen concentration in the exhaust gas and provide feedback to the computer for control of the air-fuel ratio.

When the air-fuel ratio becomes LEAN, the oxygen concentration in the exhaust increases and the oxygen sensor informs the ECM of the LEAN condition (small electromotive force: 0 volts).

When the air-fuel ratio is RICHER than the stoichiometric air-fuel ratio the oxygen concentration in the exhaust gas is reduced and the oxygen sensor informs the ECM of the RICH condition (large electromotive force: 1 volt).

The ECM judges by the electromotive force from the oxygen sensor whether the air-fuel ratio is RICH or LEAN and controls the injection time accordingly. However, if malfunction of the oxygen sensor causes output of abnormal electromotive force, the ECM is unable to perform accurate air-fuel ratio control.

The heated oxygen sensors include a heater which heats the Zirconia element. The heater is controlled by the ECM. When the intake air volume is low (the temperature of the exhaust gas is low) current flows to the heater to heat the sensor for accurate oxygen concentration detection.

Oxygen Sensor Functional Test:

1. Make sure the vehicle is at normal operating temperature. Run the engine at 2500 rpm for 3 minutes with the transmission in PARK (A/T) or NEUTRAL (M/T).
2. With engine OFF, connect the negative lead of the DVOM to engine ground. Set the DVOM to read DC VOLTS. Disconnect the oxygen sensor connector. Connect the positive lead of the DVOM to the oxygen sensor.
3. Create a full lean condition, by removing a large vacuum hose, for 10 seconds. Voltage should decrease to 0.2 volt or less and remain there.
4. Create a full rich condition, by introducing propane into intake manifold, for 10 seconds. Voltage should increase to 0.8 volt or less and remain there.
5. Verify that engine is at operating temperature and O2 sensor heating element (if equipped) is functioning. See, sensor heating element test.
6. If engine is at operating temperature and heating element passes, but the sensor fails to vary according to the test steps above, the sensor is defective.
7. Connect the oxygen sensor and clear codes.

NOTE: Only use a high input impedance Digital Volt Ohm Meter (DVOM) when testing components on computerized vehicles.

CAUTION: Verify DVOM is set to read VOLTS only (not ohms). Verify external voltage is not applied to the sensor lead, as damage to the sensor may occur.

As always, check the service manual for the proper procedures and specifications for your particular vehicle.