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Oxygen Sensors: Heart of Mixture Control

Gear case (3)Oxygen sensors were developed by the Bosch company during the late 1960s. The sensor element in the tip was made of zirconia ceramic coated on both the sides with a thin layer of platinum. The one wire oxygen sensor had a short service life. The factory mandate was 30,000 mile replacement. The next three wire O2 sensors were rated to 60,000 miles. In 1993, the factory removed oxygen sensor replacements from their recommended services, along with fuel filters. True that O2 sensors did not fail by the clock, but fail they did. And here’s how.

The oxygen sensor receives a .5 volt signal from the computer. The O2 sensor generates a signal itself resulting from the mixture in the exhaust, from about .1 to .9 volts. When the O2 sensor signal is below the .5 volt ECU signal, current flows out of the ECU. And the mixture is lean, or below stoichiometric. When the O2 signal is above .5 volts, current flows back towards to ECU, and the mixture is rich. The rich to lean cycle centers on about a .5% CO content.

1-1-fuel mixture 4-005Oxygen sensors would fail in a number of ways. The best case scenario was that they just quit working, and the system would revert to a default fuel map. That was not great for the catalytic converter, but seldom damaged it. Oxygen sensors would just get old and slow, usually from soot or oil deposits contaminating the ceramic and lengthening response time. Which generally made the mixture lean overall, and and caused stumbling on acceleration, but did not destroy anything. Some oxygen sensors would get stuck at a voltage. If the voltage was above .6 volts, the system would go lean, and overheating could result. If the O2 sensor voltage stuck below .5 volts, the system would go rich, and the catalytic converter was in peril. If the oxygen sensor shorted out internally, the system snowballed rich, and the catalytic converter would get red hot, and later crumble. Old oxygen sensors could also have a reduced voltage sine wave, which generally leaned out the system a bit. More importantly that low signal voltage curve caused the fuel injection system to not get lean and rich enough as it cycled, which would stop the Rhodium in a catalytic converter from reducing the oxides of nitrogen. Plainly put, the car would fail smog for high NOx.

oxygen sensors DSC07447 120x92The wideband oxygen sensor is the most recent type with 2 ceramic elements separated by an electrochemical gas pump. An electronic feedback loop controls the gas pump current to keep the output of the electrochemical cell constant, the current directly indicates the mixture. This sensor eliminates the lean-rich cycling of the older sensors which allows faster and more accurate mixture control. The wideband zirconia sensor is used in stratified fuel injection systems, and also in modern diesel engines. Failures with this type of sensor are not common, and are usually only associated with contamination, such as from a failed turbocharger. Old worn out engines are not common. Yet.

1-1-1-o2-sensor-wrench-88750a-003Oxygen sensors are one of the few components that generally are bad if there is a Diagnostic Trouble Code (DTC) for an oxygen sensor present in the ECU. A DTC for no signal can mean the wiring got damaged. Heater circuit malfunctions usually require eventual replacement, but the sensor output signal is normally unaffected. The problem with the reduced signal syndrome, typically cycling between only .35 – .7 volt, often remains undetected. The ECU aging test only reflects the response time and the switching interval, but not the signal height.

The last clue is that an oxygen sensor can be seen as a wear item, and will fail eventually just due to age. Whatever coats the inside of your exhaust system will also coat and/or contaminate the ceramics inside the oxygen sensor. Certain conditions will accelerate the demise of the oxygen sensors. Excessive oil usage is the biggest factor.

Gear case (3)