Oxygen Sensor: Testing and Inspection

FAULT INDICATIONS (CODE 2-1-2)
A fault code is stored if the oxygen sensor signal is:
^ Constantly below .09 VDC or above 1.1 VDC.
^ Does not vary from its mid-point value (.5 VDC) for 3 minutes after engine temperature exceeds 75° C and a programmed load threshold is maintained.

CHECK OXYGEN SENSOR SIGNAL
1. Start engine and warm to operating temperature.
2. Fold back sensor lead boot at oxygen sensor.
3. Connect voltmeter between sensor lead and ground. Voltmeter should oscillate between 0.1 and 0.9 VDC.
4. If reading is fixed at 0.4 - 0.6 VDC, check for open circuit between sensor and injection control module.
5. If reading is too low, check for grounded signal wire, intake leak, or heater element operation.
6. If reading is too high, check for signal lead short to other wiring.
7. If wiring is OK, replace sensor and recheck.

CHECK HEATER ELEMENT VOLTAGE SUPPLY
1. With engine running, check voltage between oxygen sensor terminal 1 (+) and ground.
2. Voltmeter should indicate battery voltage.
3. If no voltage is present, check lead between main relay terminal 3 and sensor connector terminal 1.
4. If voltage is present, check sensor heater signal.

CHECK HEATER SIGNAL LEAD
1. Switch ignition "ON."
2. Disconnect heater connector.
3. Check voltage between connector terminal 2 (+) and ground.
4. Voltmeter should indicate 2 VDC.
5. If voltmeter indicates 0 VDC, check resistance of signal lead to ground.
6. If voltmeter indicates 5 VDC, check lead between injection control module #A28 and sensor connector terminal 2.
7. If reading is correct, check heater element resistance.

CHECK SENSOR HEATER RESISTANCE
1. With ignition "OFF", check resistance between oxygen sensor terminals 1 and 2.
2. Ohmmeter should indicate 3 - 13 Ohms depending on resistor temperature.
3. If resistance is not as specified, replace the sensor.
4. If resistance is correct, remove oxygen sensor and inspect for physical damage.