Lovejoy

Replacing which O2 sensor would (possibly) get me better gas mileage? I have over 104K on the clock and would assume that a new one couldn't hurt.

Lovejoy

Daniel B. Martin

Replacing your Oxygen Sensors will improve performance only if they are defective. Replacing good sensors gains you nothing. There are test instruments made specifically for testing Oxygen Sensors. In addition, any technician with an automotive oscilloscope can test your sensors.

Some Nissan owners report Oxygen Sensor failures as early as 30K miles. In my own experience they last much longer than that.


There are two degrees of failure.

1) The Oxygen Sensor is "dead".
The signal from the Oxygen Sensor is outside the normal range, or completely absent. The Engine Control Module (the computer) turns on the Malfunction Indicator Light and stores one or more Diagnostic Trouble Codes. This failure mode is obvious because of the dashboard warning light.

A dead Oxygen Sensor is a severe failure and should be repaired promptly.

2) The Oxygen Sensor is "tired".
The Oxygen Sensor still works but it responds slowly to changes in the oxygen concentration in the exhaust stream. There is no MIL or DTC. This failure mode is less obvious. The symptoms are increased fuel consumption, decreased engine power, and a feeling of sluggishness. The throttle response is not crisp, like it was when the car was new. An Oxygen Sensor may become tired due to high mileage or being "poisoned" by exposure to various substances including leaded fuels, anti-freeze, anti-seize, RTV sealants, and the residue of burned engine oil. A "tired" Oxygen Sensor is still doing its job, but not performing well.

A tired Oxygen Sensor is not a severe failure and the car may be operated safely.


You may have your Oxygen Sensor(s) tested. There are electronic testers made for this specific purpose. In addition, a technician with an automotive oscilloscope can test Oxygen Sensors. The signal from a healthy sensor varies from a low threshold (around 0.2 volts) to a high threshold (around 0.8 volts), and it sweeps back and forth between those extremes about 10 times per second. If the scope trace shows the Oxygen Sensor signal failing to reach either extreme, or oscillating between them at a frequency slower than 10 cycles per second, that is evidence of a "tired" sensor.

I've read that a bad Oxygen Sensor can sometimes be rejuvenated by removing it, clamping it in a vise, and heating the sensing tip red hot with a propane torch. This is said to burn off a crust of deposits which interfere with its operation. I've never tried this torch trick myself, so cannot vouch for it's effectiveness. If you have a sensor which is already dead, you have little to lose.

For more information about Oxygen Sensors see: http://www.volksparts.com/o2sensors.htm
http://members.aol.com/carleyware/library/o2sensor.htm