Oxygen Sensor: Testing and Inspection

HEATED OXYGEN SENSOR



OPERATION
- The heated oxygen sensor functions to detect the concentration of oxygen in the exhaust gas; it converts those data to voltage, and inputs the resulting signals to the engine control module.
- If the air/fuel mixture ratio is richer than the theoretical air/fuel mixture ratio (i.e., if the concentration of oxygen in the exhaust gas is low), a voltage of approximately 1 V is output; if the air/fuel mixture ratio in leaner than the theoretical air/fuel mixture ratio (i.e., if the concentration is dense), a voltage of approximately 0 V is output.
- The engine control module, based upon those signals, regulates the amount of fuel injection so that the air/fuel mixture ratio becomes the theoretical air/fuel mixture ratio.
- Battery positive voltage is supplied, by way of the MFI relay, to the heated oxygen sensor heater. As a result, the sensor element is heated by the heater, so that the heated oxygen sensor shows excellent response even if the temperature of the exhaust gas is low.

TROUBLESHOOTING HINTS
Hint 1:
The exhaust gas purification performance will worsen if there is a malfunction of the heated oxygen sensor.
Hint 2:
If the heated oxygen sensor output voltage is outside the standard value even though the results of the checking of the heated oxygen sensor are normal, the cause is probably a malfunction of a component related to mixture control.

[Examples]
1. Malfunction of injector
2. Air is drawn into the intake manifold from a leaking gasket, etc.
3. Malfunction of volume air flow sensor, intake air temperature sensor, barometric pressure sensor or engine coolant temperature sensor.

INSPECTION

Using Scan Tool













HARNESS INSPECTION





STEP 1. Check for continuity between the heated oxygen sensor and the MFI relay.
- MFI relay connector: Disconnected
- Heated oxygen sensor connector: Disconnected

NOTE:
- Touch the ohmmeter probes to both ends of the harness.

OK: GO TO STEP 2
NG: Repair the harness. (A1 - C3, B1 - C3)





STEP 2. Check for an open circuit or a short-circuit to ground between the heated oxygen sensor and the engine control module.
- Heated oxygen sensor connector: Disconnected
- Engine control module connector: Disconnected

OK: GO TO STEP 3
NG: Repair the harness. (A4 - 76, B4 - 79)





STEP 3. Check for an open circuit or a short-circuit to ground between the heated oxygen sensor and the engine control module.
- Heated oxygen sensor connector: Disconnected
- Engine control module connector: Disconnected

OK: GO TO STEP 4
NG: Repair the harness. (A3 - 34, B3 - 42)





STEP 4. Check for continuity in the ground circuit.
- Heated oxygen sensor connector: Disconnected
- Engine control module connector: Disconnected

OK: STOP
NG: Repair the harness. (A2 - 72, B2 - 72)




SENSOR INSPECTION
1. Disconnect the connector to the heated oxygen sensor, then use the special tool (test harness) to make connections with the connection on the heated oxygen sensor side.
2. Check that there is continuity between terminal (1) (red clip of the special tool) and terminal (3) (blue clip) of the heated oxygen sensor connector [approx. 12 ohms at a temperature of 20°C (68°F)].
3. If there is no continuity, replace the heated oxygen sensor.





4. Warm up the engine until the coolant temperature is 80°C (176°F) or higher.
5. Using jumper wires, connect terminal (1) (red clip of the special tool) and terminal (3) (blue clip) of the heated oxygen sensor with the positive battery terminal and negative battery terminal respectively.

Caution:
When connecting the jumper wires, be careful not to connect them to the wrong terminals, since this could damage the heated oxygen sensor.

6. Connect a digital voltmeter to terminal (2) (black clip of the special tool) and terminal (4) (white clip).
7. While repeatedly racing the engine, measure the heated oxygen sensor's output voltage.





Standard Value:

8. If the measurements are not as specified, the cause is probably a malfunction of the heated oxygen sensor.