O2 Sensors: FYI

(Reprinted with permission from Babcox’s Tomorrow’s Technician Magazine
— May 2007, http://www.babcox.com)

How many oxygen sensors are on today’s engines?

ANSWER: It depends on the model year and type of engine. On most four- and straight six-cylinder engines, there is usually a single oxygen sensor mounted in the exhaust manifold. On V6, V8 and V10 engines, there are usually two oxygen sensors, one in each exhaust manifold. This allows the computer to monitor the air/fuel mixture from each bank of cylinders. When displayed on a scan tool, the right and left oxygen sensors are typically labeled “Bank 1, Sensor 1” and “Bank 2, Sensor 1.”

On later-model vehicles with OBD II (some 1993 and ‘94 models, and all 1995-and-newer models), one or two additional oxygen sensors are also mounted in or behind the catalytic converter to monitor converter efficiency. These are referred to as the “downstream” O2 sensors, and there will be one for each converter if the engine has dual exhausts with separate converters.

On a scan tool, the downstream sensor on a four- or straight six-cylinder engine with single exhaust is typically labeled “Bank 1, Sensor 2.” On a V6, V8 or V10 engine, the downstream O2 sensor might be labeled “Bank 1 or Bank 2, Sensor 2.” If a V6, V8 or V10 engine has dual exhausts with dual converters, the downstream O2 sensors would be labeled “Bank 1, Sensor 2” and Bank 2, Sensor 2.” Or, the downstream oxygen sensor might be labeled Bank 1, Sensor 3 if the engine has two upstream oxygen sensors in the exhaust manifold (some do to more accurately monitor emissions). It’s important to know how the O2 sensors are identified because a diagnostic trouble code that indicates a faulty O2 sensor requires that sensor to be replaced. Bank 1 is usually the front bank of cylinders on a transverse mounted V6 engine. But on a longitudinal V6, V8 or V10, it could be either the right or left bank. It may therefore be necessary to refer to the vehicle service literature to determine how the cylinder banks and oxygen sensors are labeled.


I was wondering – how does a downstream O2 sensor monitor converter efficiency?

ANSWER: A downstream oxygen sensor in or behind the catalytic converter works exactly the same as an “upstream” O2 sensor in the exhaust manifold. The sensor produces a voltage that changes when the amount of unburned oxygen in the exhaust changes. If the O2 sensor is a traditional zirconia type sensor, the voltage output drops to about 0.2 volts when the fuel mixture is lean (more oxygen in the exhaust). When the fuel mixture is rich (less oxygen in the exhaust), the sensor’s output jumps up to a high of about 0.9 volts. The high or low voltage signal tells the PCM the fuel mixture is rich or lean.

On some newer vehicles, a new type of “wideband” oxygen sensor is used. Instead of producing a high or low-voltage signal, the signal changes in direct proportion to the amount of oxygen in the exhaust. This provides a more precise measurement for better fuel control. These sensors are also called “air/fuel ratio sensors” because they tell the PCM the exact air/fuel ratio, not just a rich or lean indication like a conventional O2 sensor.

The OBD II system monitors converter efficiency by comparing the upstream and downstream oxygen sensor signals. If the converter is doing its job and is reducing the pollutants in the exhaust, the downstream oxygen sensor should show little activity (few lean-to-rich transitions, which are also called “crosscounts”). The sensor’s voltage reading should also be fairly steady (not changing up or down), and average 0.45 volts or higher.

If the signal from the downstream oxygen sensor starts to mirror that from the upstream oxygen sensor(s), it means converter efficiency has dropped off and the converter isn’t cleaning up the pollutants in the exhaust. The threshold for setting a diagnostic trouble code (DTC) and turning on the Malfunction Indicator Lamp (MIL) is when emissions are estimated to exceed federal limits by 1.5 times.

If converter efficiency has declined to the point where the vehicle may be exceeding the pollution limit, the PCM will turn on the Malfunction Indicator Lamp (MIL) and set a diagnostic trouble code. At that point, additional diagnosis may be needed to confirm the failing converter. If the upstream and downstream O2 sensors are functioning properly and show a drop off in converter efficiency, the converter must be replaced to restore emissions compliance. The vehicle will not pass an OBD II emissions test if there are any converter codes in the PCM.

I would like to know, what’s the difference between a “heated” and “unheated” oxygen sensor?

ANSWER: Heated oxygen sensors have an internal heater circuit that brings the sensor up to operating temperature more quickly than an unheated sensor. An oxygen sensor must be hot (about 600 to 650 degrees) before it will generate a voltage signal. The hot exhaust from the engine will provide enough heat to bring an O2 sensor up to operating temperature, but it may take several minutes depending on ambient temperature, engine load and speed. During this time, the fuel feedback control system remains in “open loop” and does not use the O2 sensor signal to adjust the fuel mixture. This typically results in a rich fuel mixture, wasted fuel and higher emissions.

By adding an internal heater circuit to the oxygen sensor, voltage can be routed through the heater as soon as the engine starts to warm up the sensor. The heater element is a resistor that glows red hot when current passes through it. The heater will bring the sensor up to operating temperature within 20 to 60 seconds depending on the sensor, and also keep the oxygen sensor hot even when the engine is idling for a long period of time.

Heated O2 sensors typically have two, three or four wires (the extra wires are for the heater circuit). Note: Replacement O2 sensors must have the same number of wires as the original, and have the same internal resistance.

The OBD II system also monitors the heater circuit and will set a trouble code if the heater circuit inside the O2 sensor is defective. The heater is part of the sensor and cannot be replaced separately, so if the heater circuit is open or shorted and the problem is not in the external wiring or sensor connector, the O2 sensor must be replaced.