Wide Band Oxygen Sensor Operation and Diagnosis

It is hard to believe, but the wide band oxygen sensor has been found in production vehicles for over a decade. As opposed to a conventional sensor that is accurate only at stoichiometric, the wideband sensor is accurate through air / fuel ratios of 11:1 to 20:1. As with any modern oxygen sensor, an internal heater ensures operational temperature of the sensor is achieved quickly.

At the OEM level, the wideband sensor has become a useful tool in the pursuit of reduced fuel consumption. At the other end of the spectrum, the tuning community has found these devices handy for custom ECM programming. To understand what the data stream is telling the technician, a basic understanding of how the wide band sensor functions is required. The wide band sensor shares technology with a zirconium sensor, but with some additional internal components.

A conventional oxygen sensor (Nernst cell) contains a zirconium-oxide ceramic layer. On one side of this ceramic layer is a reference sample of oxygen. The other side of the ceramic layer is exposed to exhaust gas. When the oxygen level in the exhaust is high, like a lean air / fuel mixture, the difference in oxygen concentration between the sample of air and the exhaust gas is small and a low voltage is produced. At the other end of the scale, when there is a large difference in the oxygen concentrations, there is a greater ionic movement and a higher voltage is produced. Expected voltage range for a zirconium sensor is approximately .050 volts to .950 volts.

So, on a properly functioning sensor, low voltage indicates a high oxygen content in the exhaust. Keep in mind that a high oxygen content doe not always indicate a lean mixture. A leaking exhaust or a misfire can cause the oxygen levels to go up.

In contrast, This capability of the Nernst Cell is utilized as part of the wideband sensor.

A component called a pump cell is attached to the Nernst cell. When a voltage (polarity sensitive) is provided to the pump cell, the pump cell has the ability to attract or repel oxygen. There is a small passageway that will allow exhaust gas into the chamber between the pump cell and the Nernst cell. When current is applied to the pump cell, it will increase or decrease the quantity of oxygen in the chamber. This will cause the Nernst cell to change voltage output.

Construction of a Wide Band Sensor

Figure 1 – Construction of sensor | Courtesy – Robert Bosch Corporation

Operating strategy of the engine controller aims to keep the Nernst cell at a constant 450mV. Lean exhaust gas has an excess of oxygen, and the Nernst cell initially will have a voltage lower than 450mV. To compensate for this situation, the control module will create a positive current in the pump cell. This will remove oxygen from the diffusion gap and allow the Nernst cell to produce 450mV. If the exhaust gas is rich, there is a lack of oxygen in the diffusion gap and the Nernst cell will have a voltage higher than 450mV. The engine control module will now send current in the opposite direction (negative current) to the pump cell. This will attract more oxygen in to the diffusion gap and lower the Nernst cell voltage to 450mV. The pump current becomes the control module’s method of measuring lambda or air fuel ratio.

A Lambda reading of 1 equals stoichiometry, or a 14.7:1 air / fuel ratio. If the current is positive, the air fuel ratio is lean (higher than 1.0 lambda) and if the current is negative, the air fuel ratio is rich (lower than 1.0 lambda). The relevant pump current forms the output signal and indicates air / fuel ratio. Some manufacturers will provide current flow and direction within the scan data.

Pump Current versus Lambda

Figure 2 – Pump current versus Lambda | Courtesy – Robert Bosch Corporation

The majority of manufacturers will provide a Lambda value through the data list in the scan tool. However if a voltage number is provided, the number is based upon the pump cell current going through a calibrated resistor. Fresh air (engine off) will provide approximately a 4.00 volt reading and a stoichiometric air fuel mixture (engine running) should provide approximately a 2.50 volt reading.

Voltage Output versus Lambda

Figure 3 – Voltage output versus Lambda

A quick and easy check to verify proper operation of the wide band oxygen sensor is to consult the scan data from a downstream oxygen sensor, before the catalytic converter has warmed to operating temperature. For example, if the downstream oxygen sensor consistently reads well below 450mV, this could indicate a lean condition.

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