@hamad ahsan, Impedance of O2 sensor varies according to temperature,but in cold state meter shows infinity.Warm up the engine and check the voltage at the connector of O2 sensor when engine rpm is about 2000, ( multi meter's positve lead to be connected to the wire attached with sensor and negative lead to negative terminal of battery or any metal parts of body or engine ) at this time multi meter should show 0.45 volt.If reading differs,it means sensor is bad.Digital multi meter must be used for this measurement.Read the following article it may be useful to u,
OXYGEN SENSOR: INSPECTION AND TESTING
O2 sensor usually performs unnoticed for at least 50,000 to 60,000 miles. If you suspect your sensor has failed, or is not operating up to specifications, there are several inspections and tests that can be performed before laying out the money for a new sensor.
The first point of inspection should be the connector for the sensor. Because an O2 sensor's output is a very small voltage (1 volt or less), any problem at its connection can cause the output to be erroneous. Make sure this connection is free from corrosion, grease. or any other foreign substance. Also check that the sensor's wires are not damaged, especially from heat from the exhaust manifold.
The O2 sensor, which looks like an overweight spark plug can be found between the exhaust manifold and the catalytic converter Once again carry out an inspection, this time of the area surrounding the sensor An oxygen sensor must have a free flow of ambient air to com pare with the exhaust gas in order to function properly. If a sensor is blocked by mud or other trash and can not `breathe" it will not function.
With the inspections completed, test the output of the sensor with your Digital Multi-Meter. Don't make the mistake of using an older generation analog meter to test the sensor, as possible damage to the electronics of the DME computer and the O2 sensor can result.
There will either be a one, three, or four wire O2 sensor. With only one wire, there's no problem determining which wire is the signal wire. If it is a three or four wire system and you don't have an Electronic Trouble Shooting manual to look up the details of the wires, there is a simple method to determine which wire is the signal wire. Place your DMM in the DC volt position and attach the black common test lead to a good chassis ground. Then find the O2 sensor's connector and disconnect it from the car's computer With the engine running, use the red test lead (positive) to probe the wires of the O2 sensor. Only one wire should have an output and it should be under one volt.
Once you find the correct wire, stop the engine and reconnect the sensor You will then need to break out the signal from the sensor with it still connected to the computer. You can spliced in a short test wire which enables you to test the sensor at any time. You could also use a test lead end that has a small alligator clip with a piercing tooth that can penetrate the wire's insulation.
With the sensor hooked up to the meter, restart the engine and let it warm up to normal operating temperature. Set the meter to DC volts and then set it to use the four volt range. If your meter has the ability to record minimum,maximum, and average voltage readings. Use this feature as it allows you to record and save the O2 sensor's signal the whole time it's under test. With the engine at temperature, run the throttle up to 2000 RPM and hold it for 30 seconds and then release it. Goose the throttle once and then press the meters hold button. This locks in all the voltage readings captured during the test. If your sensor is working properly, the minium voltage should be under 0.200 DC volts, the maximum at least 0.700 DC volts and the average close to 0.450 DC volts. A rich running engine would show a much higher average. Use the chart as a simple guide for diagnosing oxygen sensors.
OXYGEN SENSOR TEST RESULTS
Min Max Average TestResults ------- ------- ------- ----------------------------------
200 700 400-500 Oxygen Sensor is OK
]200 NA 400-500 Replace Sensor
NA 700 400-500 Replace Sensor
<200 700 400 Running lean
<200 <700 <400 System is lean. Enrichen mixture to see if the sensor reacts, if not, replace sensor.
<200 700 ]500 Running rich
]200 ]700 ]500 System is rich. Lean mixture to see if the sensor reacts,if not, replace sensor. All numbers in milli volts (mV).
Another important test to consider is how fast a sensor reacts to a mixture change. Check this by forcing the engine lean (remove the oil filler cap during engine operation), and then rich (inject propane into the air inlet, or restrict the flow of intake air). A properly functioning sensor should change voltages instantly. Faulty sensors generally lock onto one voltage and do not cycle up and down. Shorted sensors will read 0 volts.
Of course, it goes without saying, you should never condemn the O2 sensor unless you're sure the rest of the engine is running properly. There are many engine problems could point out a leaky fuel injector would cause a higher then average O2 sensor reading, a torn intake hose would cause a lean mixture, etc.
One final caution, some people are under the impression that disconnecting the O2 sensor will benefit them and give them more power This is not the case. The sensor is a very important component in the engine management system and is there to fine tune the fuel injection system and give you an optimum running engine.