Heated Oxygen Sensor

HEATED OXYGEN SENSOR













OPERATION
- The heated oxygen sensor senses the oxygen concentration in exhaust gas, converts it into a voltage and inputs it to the engine control module.
- The heated oxygen sensor outputs about 1 V when the air-fuel ratio is richer than the theoretical ratio and outputs about 0 V when the ratio is leaner (higher oxygen concentration in exhaust gas).
- The engine control module controls the fuel injection ratio based on this signal so that the air-fuel ratio may be kept at the theoretical ratio.
- The battery voltage is supplied to the heated oxygen sensor through the MFI relay. Therefore, the sensor element is heated by the heater so that the heated oxygen sensor remains responsive even when the exhaust temperature is low.

TROUBLESHOOTING HINTS
Hint 1:
Poor cleaning of exhaust gas will result if the heated oxygen sensor fails.
Hint 2:
If the heated oxygen sensor checked good but the sensor output voltage is out of specification, troubles of parts related to air-fuel ratio control system are suspected.
[Examples]
1. Faulty injector
2. Air leaking into the intake manifold through gasket gap, etc.
3. Faulty volume air flow sensor, intake air temperature sensor, barometric pressure sensor, engine coolant temperature sensor

INSPECTION
Using Scan Tool
















HARNESS INSPECTION




Measure the power supply voltage of the heated oxygen sensor.
- Connector: Disconnected
- Ignition switch ON
Voltage (V): Battery positive voltage
OK: GO TO STEP 2
NG: Repair the harness. (A1, B1 C1, D1 - MFI relay)




Check for an open-circuit, or a short-circuit to ground, between the engine control module and the heated oxygen sensor
- Heated oxygen sensor connector: Disconnected
- Engine control module connector: Disconnected
OK: GO TO STEP 3
NG: Repair the harness. (A4-74) (B4-72) (C4-71) (D4-73)




Check for an open-circuit, or a short-circuit to ground, between the engine control module and the heated oxygen sensor.
- Heated oxygen sensor connector: Disconnected
- Engine control module connector: Disconnected
OK: GO TO STEP 4
NG: Repair the harness. (A3-27) (B3-4) (C3-3) (D3-26)




Check for continuity of the ground circuit.
- Connector: Disconnected
OK: STOP
NG: Repair the harness. (A2, B2, C2, D2-49)

HEATED OXYGEN SENSOR CHECK





1. Disconnect the heated oxygen sensor connector and connect the special tool, Test Harness, to the heated oxygen sensor connector.
2. Check that there is continuity [11 - 18 Ohm at 20° C (680° F)] across terminals (1) and (3) of the heated oxygen sensor connector.
3. If there is no continuity, replace the heated oxygen sensor.




4. Warm up the engine until the engine coolant temperature becomes 80° C (176° F) or higher.
5. Using jumper wires connect terminals (1) (red clip of the special tool) and (3) (blue clip) of the heated oxygen sensor connector to battery (+) and (-) terminals respectively.

CAUTION: Ensure that the jumper wires are connected correctly, as wrong connections result in a broken heated oxygen sensor.

6. Connect a digital voltmeter across terminals (2) (black clip of the special tool) and (4) (white clip).




7. Race the engine repeatedly and measure the output voltage of the heated oxygen sensor.
8. If the measurements are not as specified, defective heated oxygen sensor is suspected.

NOTE: For removal and installation of the heated oxygen sensor (front) refer to Exhaust Pipe Main Muffler and Catalytic Converter.





1. Disconnect the heated oxygen sensor (rear) connector and connect the special tool (test harness) to the connector on the heated oxygen sensor (rear) side.
2. Make sure that there is continuity [11 - 18 Ohm at 20° C (680° F)] between terminals No.2 (red clip of the special tool) and terminal No.4 (blue clip) on the heated oxygen sensor (rear) connector.
3. If there is no continuity replace the heated oxygen sensor (rear).

NOTE:
1. If the scan tool does not display the standard value although no abnormality is found by the above mentioned continuity test and harness check, replace the heated oxygen sensor (rear).
2. For removal and installation of the heated oxygen sensor (rear), refer to Exhaust Pipe, Main Muffler and Catalytic Converter.