Nitrogen Oxides (NOx) Sensors

There are two NOx sensors located in the exhaust system. The nitrogen oxides bank 1, sensor 1 (NOx11) sensor is located upstream of the selective catalytic reduction (SCR) catalyst. The nitrogen oxides bank 1, sensor 2 (NOx12) sensor is located downstream of the SCR catalyst.

Each NOx sensor is equipped with a memory component which stores gain and offset characteristics of the sensor to compensate for part to part variation of the element during the manufacturing process.

The NOx11 sensor is only used to detect the presence of NOx concentrations in the exhaust system. The NOx11 sensor has two measurement chambers. The first measurement chamber is for oxygen (O₂ ) concentration and is not used. The NOx concentration measurement takes place in the second measurement chamber. The exhaust gas passes from the first measurement chamber through a second diffusion barrier into the second measurement chamber. The NOx present in the second measurement chamber is dissociated into nitrogen (N₂ ) and O₂ . The excess O₂ is pumped out of the measurement chamber by the pumping current. The amount of current required to pump the oxygen ions out of the measurement chamber calculates the NOx content. The calculated NOx content is the output from the pumping current controller in the NOx11 module and not a signal directly from the NOx11 sensor.

The NOx12 sensor detects the presence of O₂ and NOx concentrations in the exhaust system. The NOx12 sensor uses two measurement chambers to determine O₂ and NOx concentrations. The O₂ concentration is measured in the first measurement chamber. The exhaust gas enters the first chamber through a diffusion barrier. The NOx12 sensor infers an air to fuel ratio relative to the stoichiometric air to fuel ratio by balancing the amount of oxygen pumped in or out of the measurement chamber. As the exhaust gases become richer or leaner, the amount of oxygen that must be pumped in or out to maintain a stoichiometric air to fuel ratio in the measurement chamber varies in proportion to the air to fuel ratio. The amount of current required to pump the oxygen in or out of the measurement chamber calculates the air to fuel ratio. The calculated air to fuel ratio is the output from the pumping current controller in the NOx12 module and not a signal directly from the NOx12 sensor. The NOx concentration measurement takes place in the second measurement chamber. The exhaust gas passes from the first measurement chamber through a second diffusion barrier into the second measurement chamber. The NOx present in the second measurement chamber is dissociated into nitrogen (N₂ ) and O₂ . The excess O₂ is pumped out of the measurement chamber by the pumping current. The amount of current required to pump the oxygen ions out of the measurement chamber calculates the NOx content. The calculated NOx content is the output from the pumping current controller in the NOx12 module and not a signal directly from the NOx12 sensor.

On some vehicles, the NOx module and the NOx sensor are an assembly.

Fig 1: Typical NOx Sensor

Courtesy of FORD MOTOR CO.