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When Should You Replace an Oxygen Sensor?

April 1, 2007

Is there a mileage or time requirement for oxygen sensor replacement?

Simple answer: No.

Oxygen sensors were first used for fuel trim and emissions in the late 1970s and into the mid 1990s. A single sensor was installed into the exhaust stream to modify fuel delivery and maintain catalytic converter efficiency.

Beginning January 1, 1996, OBD II became a global requirement. The pre- and post-catalytic converter oxygen sensors are part of these requirements. The pre-catalytic converter oxygen sensor is used for fuel trim and the post-catalytic converter oxygen sensor is used to monitor converter efficiency.

The current Federal OBD II warranty period for an oxygen sensor is two years or 24,000 miles, whichever comes first, but with proper care and the right fuel diet, the oxygen sensor should be a maintenance-free emissions system component.

Knowing how the oxygen sensor operates and what makes up the exhaust gasses that flow past the sensors and through the catalytic converter can help in determining when to replace and prevent future problems.

Oxygen Sensor 101
Did you ever wonder where a chemistry or physics class could come in handy? The knowledge from these studies can help understand a problem with a fuel delivery system. The oxygen sensor was originally called a Lambda sensor.

The sensor is made of Zirconium Oxide (ZrO2), a chemical compound used to form the sensor’s thermal-driven electrochemical fuel cell. The Greek letter Lambda is used to describe the voltage range of the sensor when it compares the quantity of oxygen in the exhaust relative to oxygen in the atmosphere. Two Platinum (Pt) electrodes are placed on the ZrO2 to provide a connection for output voltage to a control module. An output voltage of 0.2 V (200 mV) DC represents a lean mixture where there is oxygen in the exhaust stream. A reading of 0.8 V (800 mV) DC represents a rich mixture where there is little or no oxygen in the exhaust stream. The ideal point is 0.45 V (450 mV) DC; this is where the quantities of air and fuel are in the optimum ratio, which is called stoichiometric.

The controller uses 450mV as a midpoint in a voltage range to control fuel trim for the injector pulse cycle. The sensor’s analog input to the controller is converted to a digital rich or lean command to drive a fuel trim software program. Sometimes referred to as “Block Learn,” it adjusts the cycle time of the fuel injector. The voltage generated by the sensor must be greater or less than the voltage of the damping zone to send a rich or lean signal to the controller.

The damping zone acts like a shock absorber on a suspension to prevent the voltage signal from oscillating.

A Planar Air Fuel Sensor is a combination of a standard Zirconium Oxide Oxygen sensor and a Pump Cell to maintain a constant sensing of a stoichiometric air fuel ratio through the extreme rich and lean conditions. The pump cell is a diffusion gap in the Zirconium Oxide of the sensor that is connected to a control circuit.

The pump cell controls the oxygen concentration of the sensor by adding or subtracting oxygen to the diffusion gap. Input to the electronic circuit modifies the oxygen concentration by changing the polarity of the current flow in the pump cell. The changing polarity of the input and trim current flow causes the control circuit to send a rich or lean signal to the engine control module.

How long should an oxygen sensor last?
An oxygen sensor should outlast the vehicle emissions warranty. Manufacturers recommend that the unheated type used from the late 1970s to the 1990s be inspected every 30,000 miles and the heated type used from the early 1980s to the mid 1990s be inspected every 60,000 miles. Manufacturers for current generation of sensors starting in the mid 1990s should be inspected every 100,00 miles.

In fact, with proper powertrain maintenance, it is possible for the sensor to last the life of the vehicle, which could be in excess of 250,000 miles.

When does an oxygen sensor need to be replaced?
The service engine soon light will come on and a diagnostic trouble code(s) will be stored. The following is a partial list of Diagnostic Trouble Codes (DTC) for the pre (Sensor 1) and post (Sensor 2) sensors. If the sensor is damaged or not responding, it should be replaced. There are DTCs for V type engines (Bank 1 and 2) and a third sensor.

There may be multiple codes stored for a sensor. A fuel delivery malfunction can be the reason for an oxygen sensor to fail. A fuel injector malfunction could be the root cause for the sensor failure. Just replacing the sensor may not be a long-term solution.

Do oxygen sensors degrade over a period of time?
Simple answer: Yes. What causes an oxygen sensor to degrade? When silicon was an ingredient in RTV and coolant, the silicon could cause the sensor to rapidly degrade. It was referred to as silicon poisoning.

Today fuel and maintenance are two major contributors to a sensor’s degradation. Gasoline and diesel fuel are refined products from crude oil. The refined product contains a mixture of different hydrocarbons including Olefins, Benzene and the chemical element Sulfur.

Sulfur is a chemical element that occurs naturally in crude oil. The refining process reduces the concentration of sulfur in the gasoline. Sulfur can cause the degradation of an Oxygen sensor and the concentration of the Sulfur in the gasoline will determine the rate at which the sensor will degrade.

Gasoline with a content of 1,000 parts per million (ppm) have been shown to cause accelerated degradation resulting in the illumination of a service soon light. To put a 1,000 ppm in perspective, if you have a thousand gallons of gasoline, it will have one gallon of Sulfur. Gasoline also contains other added ingredients.

The following are ingredient descriptions for gasoline additives: Octane enhancers, antioxidants, metal deactivators, ignition controllers, icing inhibitors, detergents and corrosion inhibitors.

One of these ingredients is MBTE (methyl tertiary-butyl ether). It was originally introduced in the late 1970s as an octane enhancer to replace Tetra Ethyl Lead for catalytic converter-equipped vehicles. It is also used as an oxygenate. MBTE has little or no affect the operation of the oxygen sensor introduced in the 1980s. But, when Congress passed the Clean Air Act Amendments of 1990, MTBE levels in the new “reformulated gasoline” for certain areas of the country increased. This has affected the life of some newer oxygen sensors. Alcohol in the form of Methanol and Ethanol are oxygenates that are added to gasoline. E85 fuel is a mixture of 85% Ethanol and 15% gasoline. E85 will burn cleaner and produce less degradation of the oxygen sensor. But, it is a compromise in fuel economy, because there is less energy in a gallon of E85 than a gallon of gasoline. Methanol is a fuel associated with racing. It is a hazardous material and poisonous which limits its use as a commercial fuel.

Motor oil contains phosphorous which can also cause the degradation of the sensor when excessive oil vapor is introduced through crankcase ventilation. So one of the components that should be recommended after a oxygen sensor replacement is the PCV valve.

What can cause an oxygen sensor to fail?
The most vulnerable part is the wiring and connector.

Next is the heater. Its function is to bring the sensor to operating temperature during cold starts and engine warm up. It can be damaged by thermal shock.

Excessive heat is usually the cause for damage to the wiring. If the connector and wiring is not properly routed and secured, there is good possibility that either or both can be damaged.

Does overall engine maintenance affect the life of an oxygen sensor?
Simple answer: Yes. An important maintenance item is the oil change. It contributes to the life of an oxygen sensor. Positive Crankcase Ventilation (PCV) can contribute the degradation of the oxygen sensor. Vapors from contaminated oil in the crankcase can shorten the life of an oxygen sensor. This is a reason why you should use the manufacturer’s recommended oil for the vehicle. All petroleum products will contain sulfur.

Can an oxygen sensor affect engine performance and fuel economy?
Simple answer: Yes.

The oxygen sensor drives fuel trim and fuel trim is all about fuel economy. Good fuel and good maintenance are key to long component life and fuel economy.


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