It is first important to understand that an OBDII code in itself is not indicative of a failed oxygen sensor. Sensors simply report information. An oxygen sensor that reports a lean fuel mixture, for example, will certainly set off a code. This sensor is doing its job and does not need to be replaced.
If a failing or dead sensor is the issue, there are several OBDII codes in particular that will be set off (more on this in the following section). In turn, the vehicle itself will often display physical symptoms due to a sensor that is not functioning properly.
A decrease in fuel efficiency can be a telltale sign that an O2 sensor is not performing as it should. This can happen because of a fuel mixture that is either too lean or too rich. Such a swing in A/F ratio is a sign that an upstream or control sensor is faulty. The downstream or diagnostic sensors only monitor the exhaust leaving the catalytic converter and will not cause such an issue.
Other symptoms of a bad oxygen sensor include a rough idle, a misfire, and/ or hesitation when trying to accelerate. Keep in mind, however, that these issues can also have other causes that have no relation to the health of a vehicle’s oxygen sensors. Therefore, none of them alone is cause enough to replace one. A combination of an OBII warning, engine performance issues, and a physical inspection of the sensor is often required to make a proper diagnosis.
Oxygen sensor failure can often be traced to one of three common factors: Age and high mileage, an internal contaminant (poisoning) or an electrical issue. See this page for a list of common oxygen sensor contaminants.
One or two wire unheated oxygen sensors should be inspected or replaced every 30,000 miles. These sensors must rely solely on hot exhaust gas to reach their operating temperature, and are designed to let a large amount of exhaust make contact with the active ceramic element.
Heated oxygen sensors are less prone to contamination, as their internal heat source allows them to be placed much farther downstream than unheated sensors. Heated sensors should be inspected or replaced every 60,000 miles. While heated oxygen sensors can be placed in safer locations than unheated models, they are made up of multiple circuits that can in turn allow for electrical issues. If the heater circuit in a sensor goes bad, the sensor will not function properly. Heater circuit issues are in fact a common source of OBDII codes.
In order to do their jobs, all oxygen sensors must be exposed to a constant stream of harmful exhaust gasses, extreme heat and high velocity particulates. Because of this, their efficiency will inevitably decrease over time.
Sometimes, oxygen sensors can become contaminated by elements from within the engine. Exhaust from an overly rich fuel mixture can foul an O2 sensor, as will leaded fuel. Antifreeze or silicone residue resulting from faulty gaskets can have the same effect. The sensors pictured here have been poisoned and need to be replaced.