Oxygen sensors have been used for more than a quarter of a century, dating back to 1980 when the first computerized engine control systems appeared. Part of the fuel management system, the oxygen (O2) sensor monitors unburned oxygen in the exhaust. The powertrain control module (PCM) uses this information to determine if the fuel mixture is rich (too much fuel) or lean (not enough fuel).
To provide the best performance, fuel economy and emissions, the PCM has to constantly readjust the fuel mixture while the engine is running. It does this by looking at the signal from the O2 sensor(s), and then increasing or decreasing the on-time (dwell) of the fuel injectors to control fuel delivery.
Oxygen sensors don’t produce a signal until they’re hot, so the O2 sensors in most late-model vehicles have an internal heater that starts heating up the sensor as soon as the engine starts. Older, first-generation O2 sensors lacked this feature and took much longer to reach operating temperature, which increased cold start emissions.
Once the sensor is hot, a zirconia-type O2 sensor will generate a voltage signal that can range from a few tenths of a volt up to almost a full volt. When there is little unburned oxygen in the exhaust, the sensor usually generates 0.8 to 0.9 volts. The PCM reads this as a “rich” signal, shortens the duration of the fuel injector pulses to reduce fuel delivery, and leans out the fuel mixture.
When there is a lot of unburned oxygen in the exhaust — which may be from a lean fuel mixture, but can also occur if the engine has a misfire or compression leak — the O2 sensor will produce a low voltage signal (0.3 volts or less). The PCM reads this as a “lean” signal, increases the duration of the injector pulses, and adds fuel to enrich the fuel mixture.
A slightly different variation on this is the titania-type O2 sensor. Used in some older Nissan and Jeep applications, this type of sensor changes resistance rather than producing a voltage signal.
In recent years, the design of O2 sensors has changed. The ceramic thimble-shaped element in zirconia-type O2 sensors has been replaced by a flat strip ceramic “planar” style sensor element.
The basic operating principle is still the same (the output voltage changes as O2 levels in the exhaust change), but the new design is smaller, much more robust and faster to reach operating temperature. You can’t see the difference from the outside because the tip of the sensor is covered with a vented metal shroud, but many O2 sensors from 1997 and up use the planar design.
Another change has been the introduction of “wideband” O2 sensors, which are also called “Air/Fuel” or A/F sensors. (See the Tech Feature on pages 36-42 of this issue for additional information and diagnostic tips.)
In 1996, vehicles also began using oxygen sensors to monitor the operation of the catalytic converter. A “downstream” O2 sensor is placed either in, or just behind, the converter to monitor oxygen levels after the exhaust had reacted with the catalyst.
If the operating efficiency of the converter drops below a certain threshold that might cause an increase in emissions, it sets a diagnostic trouble code (DTC) for the converter and turns on the Check Engine Light.
First-generation O2 sensors typically have a limited service life, and may need to be replaced for preventive maintenance somewhere between 50,000 and 80,000 miles. O2 sensors on 1996 and newer vehicles typically have a much longer service life of 100,000-plus miles, and do not have to be replaced unless they’ve been contaminated or damaged.
When O2 sensors get old, they can become sluggish and slow to respond to changes in exhaust oxygen levels. Typical symptoms include a drop in fuel economy and higher exhaust emissions.
A bad O2 sensor should not affect engine starting, cause a misfire (unless the spark plugs become carbon fouled), or cause engine stalling or hesitation problems. A sluggish or fouled O2 sensor will typically read low (lean) and cause the engine to run rich.
If the heater circuit inside the O2 sensor fails, or the sensor stops producing a signal due to an internal failure or a wiring fault (a loose or corroded wiring connector), it will usually set an O2 sensor code (P0130 to P0147).
The codes can be read by plugging a scan tool into the vehicle’s diagnostic connector. But many times, other engine problems will set codes that may seem to indicate a bad O2 sensor, but in fact do not. A P0171 or P0174 lean code, for example, means the O2 sensor is reading lean all the time. The real problem may not be a bad O2 sensor, but possibly an engine vacuum leak, low fuel pressure or dirty fuel injectors that are causing the engine to run lean. An engine misfire, leaky exhaust valve or a leak in the exhaust manifold gasket that allows air into the exhaust may also cause this type of code to be set.
Ever wonder what causes O2 sensors to fail? As O2 sensors age, they slow down. But this usually isn’t a factor until the sensor has upward of 75,000 or more miles on it. So when an O2 sensor fails prematurely, the cause is often contamination.
Contaminants can come from a number of sources. If the engine has an internal coolant leak due to a crack in the combustion chamber or a leaky head gasket, and the cooling system contains a conventional antifreeze with silicate rust inhibitors (conventional green coolants do, but long-life orange coolants such as Dex-Cool do not), the silicates can pass into the exhaust and contaminate the O2 sensors.
Another source of contamination is the anti-wear ingredients in ordinary motor oil. They can be contaminated by phosphorus and zinc from motor oil if the engine has an oil consumption problem caused by worn valve guides or piston rings. However, the amount of phosphorus and zinc in motor oil has been reduced in recent years to minimize the risk of O2 sensor and catalytic converter contamination.
Every engine uses a small amount of oil, and over time the contaminants can add up. As the engine accumulates miles, and the valve guides, rings and cylinders start to wear, oil consumption goes up. Consequently, in a high-mileage engine that is using oil, phosphorus and zinc contamination of the O2 sensors and catalytic converter can be a problem.
If the O2 sensors are sluggish or have failed, they obviously need to be replaced. But replacing the O2 sensors will only temporarily restore the fuel feedback control system. Unless the oil burning is eliminated, the new O2 sensors will eventually suffer the same fate. Same for a fouled catalytic converter.
When an O2 sensor is contaminated, it may not react very quickly to changes in the air/fuel mixture. Or, the sensor may read low (lean) all of the time, causing the PCM to add too much fuel. The result is reduced fuel economy and higher emissions.
The U.S. Environmental Protection Agency says bad O2 sensors account for a high percentage of emissions test failures. So if the Check Engine light is on and there’s an O2 code or a lean code, chances are the O2 sensors may need to be replaced.