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Oxygen Sensor- 99+ Grand am - Testing and replacement

02-08-2007, 06:37 PM
Oxygen Sensor Test & replace

All 99+ models are equipped with two oxygen sensors.
One upstream Oxygen sensor, and one downstream Oxygen sensor.

The oxygen in the exhaust reacts with the inside of the oxygen sensor to produce a voltage output which varies from 0.1vlt(high oxygen lean mixture) to 0.9 vlt(low oxygen, rich mixture).
The upstream oxygen sensor, mounted in the exhaust before the catalytic converter, provides a feed back signal to the PCM the indicates the amount of oxygen in the exhaust.

The PCM monitors this varied voltage continously to determine teh required fuel injector pulse width and control air/fuel ratio. A mixture ratio of 14.7 parts air to 1 part fuel is the ideal ratio for minimum exhaust emissions, and the best fuel economy and engine performance.

The downstream oxygen sensor, mounted in the exhaust after the catalytic converter has No effect on PCM control of the air/fuel ratio. The down stream oxygen sensor is the same as teh upstream sensor and operates in the same way.
The PCM uses teh downstream signal to monitor efficiency of the catalytic converter. A downstream oxyegn sensor will produce a slower fluctuating voltage signal the reflects the lower oxygen in the post catalytic converter exhaust.

An oxygen sensor produces no voltage when it is below normal operating tempratures which is about 600deg F. During this warm up time the PCM operates in an open loop fuel control mode. When this happens the PCM controls fuel metering based on the inputs of other sensors and its own programs.

Proper operations of the Oxygen sensor depends on 4 conditions:
1- (Electrical) The low voltage generated by the sensor requires good clean connections which should be checked first when sensor troubles are suspected.
2- (Outside air) the sensor needs outside air circulation to its internal portion of the sensor. When installing a oxygen sensor make sure the air passages are not restricted.
3- (Proper operating temps) The PCM will not react to the sensor signal until the sensor reaches 600deg F. This should be considered when testing the performance of the sensor.
4- (unleaded fuel) Is important for proper operations of the sensor.

The PCM can detect several different oxygen sensor problems and will set a diagnostic trouble code(DTC) to indicate the specific fault. When a fault is detected the PCM will run in open loop fuel mode as discussed earlier.

Testing the sensors

-The oxygen sensor is a very sensative to excessive circuit loads and circuit damage.
-The safest way to test is to disconnect the O2 sensor connector and install jumper wires between the connectors and connect the voltmeter to the jumper wires.
-Performing this test will set a trouble code and illuminate the service engine light.
-A scann tool will be needed to clear the code when testing and or replacmnet is complete.

Here is a picture of the upstream O2 and its connector.

Here is a picture of the downstream O2 sensor.
(1 is the converter, 2 is the resonator)
-Connect the positive lead of the voltmeter to the Purple or Purple/White wire
and connect the negative lead to the voltmeter to the Tan or Tan/White wire
to the sensor wires.
If testing a 1 wire oxygen sensor, connect the ground to the chassis ground.

- Turn the ignition "ON" but dont start the engine.
- The meter should read bewteen 400-450 millivolts(0.40-0.45volts).
- If it doesnt check and repair the wire fromt the sensor to the PCM.

- Start the engine and allow it to warm up to normal operating temps,
and again check the voltage of the sensor signal.
- Voltage from an upstream sensor should range from 100-900 millivolts(0.1-0.9volts) and switch activly between high & low readings.
- Voltage from a downstream sensor should also read between 100-900 millivolts
but should not switch activly. The downstream sensor voltage may stay toward the center of the range (around 400millivolts) or stay realativly longer periods of time at the upper & lower limits of the range.

- Check battery voltage and circuits to the oxygen heater.
Disconnect the electrical connector and connect the Negative lead of the voltmeter to the Black wire terminal and the positive lead to the Brown wire terminal of the sensor connector.
-Turn the igntion "on" the meter should read approx 12 volts.
- If not check the power & ground circuits to the sensor, dont forget to check the fuses.

- Allow the sensor to cool down and check for resistance of the oxygen heater.
- With the connector disconnected , connect the Ohm meter to the two sensor terminals(sensor side). The oxygen sensro pigtails are ususally not color coded but the heatrer wires are usually the white wires.

- The sensor resistance shoudl be between 3.0-10.0 ohms.
- If an open circuit or excessive resistance is detected replace this oxygen sensor.
- If tests indicate the sensor is good and not hte cause of driveability problems then check the wiring harness and connectors to the PCM for a short circuit. If not problems found have the vehicle checked by the Dealer of other appropriate service department.

The exhaust pipe contract when cold and the oxygen sensor may be hard to loosen when the engine is cold.
To make removal easier start the car for 1-2 minutes, then shut if off.

BECAREFULL not to burn yourself during this procedure.
- A few guidelines to follow when replacing the O2 sensors.
a) the sensor has a perminatly attached pigtail wire which should not be removed,
nor should you damage the electrical connector, this will cause harm to the operations of the sensor.
b) Keep dirt & grease away from the electrical connector and teh louvered end of the sensor.
c) Dont use cleaning solvents of ANY kind on the sensor.
d) Dont drop or roughly handle the sensor, this will damage the sensor.

- If replacing the downstream O2 sensor, raise and support the vehicle on jsckstands.
- Disconnect the electrical connector.
- Using a suitable wrench or special O2 sensor socket, unscrew the sensor from the exhaust manifold.
O2 sensor socket pictured here.
- Anti seize coumpond must be used on the threads of the sensor for future removal.
- Most new sensor come with this already applied, if not, be sure to apply some to the threads of the sensor.

- Install the new sensor, secure it tightly and reconnect the electrical connector and lower the vehicle.

Hope that helps !

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