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19th Annual Technical Forum: Oxygen Sensors

What are Air/Fuel sensors, and how do they differ from ordinary Oxygen Sensors?


A. Air/Fuel (A/F) sensors, which are also called “wide range” or “wide band” oxygen sensors, are used on a growing number of late model vehicles to improve emissions and engine performance.


Externally, A/F sensors look the same as any other oxygen sensor. However, most A/F sensors have five wires (though some have six), and internally they are completely different. Instead of generating a simple rich or lean voltage signal when the fuel mixture changes rich or lean, an A/F sensor provides a variable current signal that allows the PCM to determine the exact air/fuel ratio of the engine. This allows A/F sensors to read much leaner air/fuel mixtures than an ordinary O2 sensor, which allows the engine computer to control emissions more accurately under a wider range of operating conditions and fuel mixtures.


Another difference is that A/F sensors operate at a much higher temperature (1,200 degrees F) than an oxygen sensor. An ordinary oxygen sensor needs to be heated to about 600 to 650 degrees F before it will generate a usable voltage signal. An A/F sensor, by comparison, needs a higher output internal heater so it can reach 1,200 degrees F to produce a signal.

Inside an A/F sensor is something called a “Nerst” cell. The Nerst cell serves as an ion pump to move oxygen in and out of a diffusion gap. This affects the reference voltage and direction of current within the sensor element to reveal the actual oxygen level in the exhaust.


Performance engine builders often use A/F sensors when dyno tuning engines because the A/F sensors reveal the exact air/fuel mixture that’s going into the engine. This allows them to tweak the fuel mixture for peak power without getting it too rich (which wastes fuel) or too lean (which can result in engine damaging detonation).

Like ordinary O2 sensors, A/F sensors can be contaminated by phosphorus if an engine is burning oil, or by silicates if it has an internal coolant leak. Contaminated sensors can be sluggish and slow to react to changes in the air/fuel mixture. Diagnosis usually requires a professional grade scan tool with software capabilities similar to a factory scan tool.


Q. Does an oxygen sensor fault code tell you which sensor is having a problem?

A. Many late model engines can have as many as four or more oxygen sensors, so it’s important to know which sensor is having a problem if there is an O2 sensor fault code. The code will usually identify the problem sensor by its number and bank. However, this can be rather confusing unless you know how the sensors are numbered, and which cylinder bank is which.

On engines with multiple oxygen sensors, the sensors will be numbered 1, 2, etc. They will also be identified on V6, V8 and V10 engines by the cylinder bank they read (Bank 1 or Bank 2). Sensor 1 is always an upstream sensor in the exhaust manifold. Sensor 2 is usually a downstream sensor behind the converter — unless the engine is an inline four or six with two upstream sensors (as is common on BMW engines). Sensor 3 or 4 would always be a downstream sensor. Bank 1 is the side that includes cylinder number one in the engine’s firing order. Bank 2 is the opposite side. If you don’t know which cylinder bank is which, look up the engine’s firing order diagram.


On most General Motors V6 and V8 engines, Bank 1 is the left cylinder bank as the engine is viewed from above or from the driver’s seat. On Ford V6, V8 and V10 engines, it’s just the opposite. Bank 1 is on the right side as viewed from above or the driver’s seat. With Chrysler engines, most rear-wheel drive V6, V8 and V10 engines have Bank 1 on the left, but on their transverse-mounted V6 engines, Bank 1 is on the right.



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