When should an oxygen sensor be replaced?
A. Only when it is necessary.  Most late model vehicles do not have a recommended replacement interval for oxygen sensors, though some oxygen sensor manufacturers do say that replacing O2 sensors at high mileage is a good way to assure peak performance and fuel economy.

Like other engine sensors, there’s no need to replace an O2 sensor as long as it is working properly and accurately reading the oxygen content of the exhaust. It is generally assumed that as long as the Check Engine light is off and there are no O2 sensor codes, the O2 sensors are probably fine. But this assumption may not be correct in the case of a sensor that is biased rich or lean, or that has become sluggish and is slow to respond to changes in the air/fuel mixture. Such problems may not be bad enough to set a code, but they could have a detrimental effect on engine performance, fuel economy and emissions.

Such problems often require professional diagnosis with a scan tool that not only displays codes, but can also read engine performance data such as loop status, fuel trim, and individual O2 sensor data. A graphing multimeter may also be needed to see how the O2 sensors are responding to changes in the air/fuel mixture. If an O2 sensor is not reading properly or is borderline, it should be replaced regardless of its age or mileage.

Of course, the average vehicle owner has no clue as to how well his oxygen sensors are working. If there’s no Check Engine light or any codes that might suggest an O2 sensor related fault, he has no way of knowing if any O2 sensors need to be replaced. The only clues may be a loss of fuel economy, or elevated carbon monoxide emissions (if a pre-OBD II vehicle fails a tailpipe emissions test).

Aging upstream O2 sensors that have become sluggish or contaminated will typically make the air/fuel mixture run rich. The engine computer (PCM) uses input from the O2 sensors to adjust the air/fuel mixture.  If the voltage output from the sensor is low (indicating a false lean condition), the PCM will make the fuel mixture richer to compensate. This increases fuel consumption and emissions.

A bad O2 sensor can also prevent some of the OBD II readiness monitors from completing their self checks. This, in turn, can cause a vehicle to be rejected if it has to take a plug-in OBD II emissions test.
A bad downstream O2 sensor (behind the catalytic converter) may prevent the catalyst monitor from running, or even cause a false P0420 catalyst efficiency code to be set.

Q. At what point does O2 sensor performance start to decline?
A.  It’s hard to say because a lot depends on the condition of the engine. If a high-mileage engine is using oil due to piston ring, cylinder or valve guide wear, phosphorus from the oil will contaminate the O2 sensors and accelerate the aging process. The same applies to coolant that may be seeping past a leaky head gasket.

As a rule, the service life of oxygen sensors in most 1996 and newer vehicles with OBD II should be 100,000 to 150,000 miles or more — assuming no problems that could cause the O2 sensors to fail prematurely.  On pre-1996 vintage vehicles, the oxygen sensors were not as robust, and typically had a service life of 50,000 to 80,000 miles.  Because of this, some of these older vehicles did have a recommended replacement interval for changing the O2 sensors.