O2 Sensors - How They Work And Why They Quit (ASN 1294-01)

O2 SENSORS - HOW THEY WORK AND WHY THEY QUIT

TECHNICAL SERVICE BULLETIN

O2 SENSORS: HOW THEY WORK & WHY THEY QUIT

Model(s): 1986-95 Acura Vehicles

Group: Fuel & Emissions

Bulletin No.: ASN 1294-01

Date: December, 1994

SERVICE INFORMATION

Many people are mystified about the workings of that device in the exhaust system called the oxygen sensor (O2S). How does something in the exhaust stream generate voltage and help the engine control module (ECM) to adjust the air/fuel mixture? What makes an O2S go bad?

The heart of a typical O2S is a thimble-shaped element made of a special material called zirconium dioxide (ZrO2). The inner and outer surfaces of this element each have an electrode made of a layer of thin porous platinum. The O2S is designed so that the inner surface of this element is exposed to ambient air (the atmosphere), while the outer surface, which also has a porous ceramic coating, is exposed to the exhaust gases. See Fig 1 .

While some oxygen is still present in burned exhaust gases, there's obviously more oxygen present in the atmosphere. Because the oxygen proportions differ between the inner and outer surfaces of the element, and thanks to the special properties of the ZrO2, a voltage is generated between the two electrodes.

Keeping in mind that the amount of oxygen in the atmosphere is relatively constant, the voltage output will vary as the amount of oxygen in the exhaust gases varies. Less voltage is generated when the air/fuel mixture is lean because of the higher oxygen content in the exhaust. More voltage is generated when the air/fuel mixture is rich because of the lower oxygen content in the exhaust. By monitoring this voltage, the ECM knows how rich or lean the air/fuel mixture is, and it adjusts the mixture accordingly.

So what makes an O2S stop working properly? Carbon from exhaust? Sounds logical, but no, the number one enemy of an O2S is silicone.

When the exhaust side of the element is exposed to silicone, the pores of the protective ceramic coating become clogged. Once this occurs, the exhaust side receives less oxygen. As a result, the 02S generates more voltage than it normally would for a given air/fuel mixture. This may cause the ECM to lean the mixture enough to cause driveability problems or trigger the malfunction indicator lamp (MIL).

If the ambient air side of the element is contaminated with silicone, the porous electrode becomes clogged, reducing the oxygen on that side. Then the O2S will generate less voltage, so the ECM tries to compensate by richening the mixture. This too may deteriorate driveability or trigger the MIL.

Sometimes the source of the silicone may be the fuel, but the more likely sources are the silicone sprays, greases, and adhesives used in most service departments these days. When you use these products, don't let them get into the engine's air intake tract, the exhaust system (upstream of the O2S), or the vents on the O2S.