How to tell if O2 sensor is bad?
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"Bad" is a relative term, as sensors go. The O² sensor can still be reporting data to the ECM, but the data may not be correct.
The O² must send a variable voltage signal to the ECM between 0.300 and 0.700 V for the ECM to enter closed-loop mode. If the sensor does not report a value within range after the coolant temperature has risen to a specified point, the ECM will store an error code and light the "SES" lamp on the dash.
However, the O² can still be "bad" by reporting a value within the acceptable range, but a value that is still incorrect. O² sensors DO degrade over time, as combustion products accumulate and the sensing element loses sensitivity. The typical failure mode for an O² sensor is to report a lower and lower voltage as they degrade, causing the ECM to interpret a lean mixture. The ECM will enrich the fuel mixture until the sesor reports correct data, however, this may be too rich in the real world. Generally, if you sensor has been in service for more than 30,000 miles, it is suspect. You can remove and test the sensor for a "GO/NO-GO" condition, but testing for correct signal and calibration is very difficult. If you suspect that there is no signal whatsoever, remove the sensor, connect a voltmeter to the lead and case ground, then expose the sensing tip to a propane torch flame. A propane flame is nearly completely devoid of oxygen, and should cause the sensor to generate a signal of at least 0.750V (750mV) within sixty seconds. When the sensor tip is removed from the flame, the generated voltage should drop quickly to less than 100mV. If the sensor cannot achieve these outputs, it is probably nearly "dead" and should be replaced.
Another indication of a failing sensor is a rich odor from the exhaust, and decreasing fuel mileage.
The O² must send a variable voltage signal to the ECM between 0.300 and 0.700 V for the ECM to enter closed-loop mode. If the sensor does not report a value within range after the coolant temperature has risen to a specified point, the ECM will store an error code and light the "SES" lamp on the dash.
However, the O² can still be "bad" by reporting a value within the acceptable range, but a value that is still incorrect. O² sensors DO degrade over time, as combustion products accumulate and the sensing element loses sensitivity. The typical failure mode for an O² sensor is to report a lower and lower voltage as they degrade, causing the ECM to interpret a lean mixture. The ECM will enrich the fuel mixture until the sesor reports correct data, however, this may be too rich in the real world. Generally, if you sensor has been in service for more than 30,000 miles, it is suspect. You can remove and test the sensor for a "GO/NO-GO" condition, but testing for correct signal and calibration is very difficult. If you suspect that there is no signal whatsoever, remove the sensor, connect a voltmeter to the lead and case ground, then expose the sensing tip to a propane torch flame. A propane flame is nearly completely devoid of oxygen, and should cause the sensor to generate a signal of at least 0.750V (750mV) within sixty seconds. When the sensor tip is removed from the flame, the generated voltage should drop quickly to less than 100mV. If the sensor cannot achieve these outputs, it is probably nearly "dead" and should be replaced.
Another indication of a failing sensor is a rich odor from the exhaust, and decreasing fuel mileage.
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