Oxygen sensor; photo courtesy PelicanParts.com
Oxygen sensor; photo courtesy PelicanParts.com. Click image to enlarge

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By Jim Kerr

Air, fuel and spark are the three ingredients needed for a gasoline engine to produce power. Other than an occasional change of spark plugs and plug wires, most contemporary vehicle ignition systems will operate their whole life without any major parts or adjustments.

While it’s easy to see fuel disappear as the gas gauge needle drops to empty, for a vehicle to operate properly, that fuel must be mixed with the correct amount of air. We refer to this as the air/fuel ratio. Too much fuel in the mix – a “rich” mixture, causes gas mileage to drop and catalytic converters to overheat. Too little fuel in the mix – a lean mixture, can cause hesitations, misfires, engine overheating and pinging or detonation, which can damage an engine.

Many sensors on a fuel-injected engine are used to measure the amount of air entering an engine. The computer can then inject the proper amount of fuel. However, even with all this sophisticated hardware it is still possible for the measurements to be off a little. This affects tailpipe emissions and engine operation.

There is one sensor that “fine-tunes” the air fuel mixture; it’s called the oxygen sensor. Oxygen sensors in front of the catalytic converter are used to fine-tune fuel delivery. Oxygen sensors located behind the catalytic converter are used to monitor the efficiency of the converter in reducing emissions and have nothing to do with air/fuel delivery. Sensors may appear the same but they do differ for their particular use, so it is important to use the correct oxygen sensor for the correct location.

Oxygen sensor; photo courtesy YotaTech.com
Oxygen sensor; photo courtesy YotaTech.com. Click image to enlarge

The oxygen sensor does just what it is called – it senses oxygen. Air entering an engine contains about 20.6 per cent oxygen. Most of that oxygen is used to support the combustion of the gasoline. In a properly running engine, only about one per cent remains, which travels out the exhaust pipe past the oxygen sensor. There are different types of oxygen sensors, but the most common types generate a voltage just like a battery. The voltage rises and falls in response to the amount of oxygen in the exhaust gases and the engine computer monitors this voltage.

When the computer sees the signal from the sensor staying low most of the time, it represents a lean mixture and the computer compensates by injecting additional fuel into the engine. If the sensor voltage is high most of the time, the computer reduces the amount of fuel injected. The oxygen sensor acts like the policeman of the fuel injection system, monitoring the results of the combustion and reporting back to the computer any errors that need correcting.

Oxygen sensors do wear out. According to Bosch, a major automotive supplier, it may take 100,000 km or more but eventually the sensors stop responding to changes in oxygen levels. This is called a degraded sensor and can cause poor fuel economy or driveability. A technician can watch the sensor voltage with a scan tool to see if it is responding properly and if it doesn’t, it needs to be replaced.

Sensors can become contaminated too. Oil deposits from worn engines, engine coolant in the exhaust or non-automotive silicone sealers can all contaminate a sensor, making replacement necessary.

Oxygen sensor; photo courtesy GMHighTechPerformance.com
Oxygen sensor; photo courtesy GMHighTechPerformance.com. Click image to enlarge

Oxygen sensor signals can be false. An air leak in the exhaust system such as a cracked exhaust manifold allows extra air past the sensor. The engine may be operating properly, but the oxygen sensor now reports a lean mixture and the computer starts injecting too much fuel. A misfiring cylinder causes the most common false signal, usually a bad sparkplug or plug wire. When a cylinder misfires, the oxygen in that cylinder is not consumed and goes out the exhaust. The oxygen sensor measures it and tells the computer to inject more fuel. Even an intermittent misfire on one cylinder can cause a significant drop in fuel economy.

Finally, many oxygen sensors have heater elements in them. An oxygen sensor has to be hot (above 350 degrees Celsius) to work. The exhaust gases will heat them but it takes time. An electric heater element can heat the sensor so it starts working only a few seconds after the engine is started. The heater is an important part of the sensor, but doesn’t do any of the sensor monitoring. It only allows it to operate sooner.

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