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A Look At O2 Sensors


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. The oxygen (O2) sensor is part of the fuel management system. It 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 are 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, or 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 The ceramic thimble-shaped element in zirconia-type O2 sensors has been replaced by a flat strip ceramic “planar” style sensor element.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. This type of O2 sensor also uses a flat strip ceramic element inside, but it has extra internal circuitry that allows the sensor to measure the exhaust air/fuel ratio with a much higher degree of precision. It can tell the PCM the exact air/fuel ratio, not just a gross rich or lean indication as other O2 sensors do.


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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