NOX sensor

A friend’s 59-reg BMW E90 330Ci coupé needed new injectors and a new NOX sensor to cure a misfire. On what principle does a NOX sensor operate? Was its failure a coincidence and was its replacement a precaution? Aren’t the Lambda sensors enough? Why don’t lesser engines need one? Robert Taylor

Depending on the initial failure, it's possible the injector fault caused sufficient damage to the NOX sensor to render it inoperative.

The NOX sensor is an addition to such engines, aimed at reducing the harmful output of unwanted gases. The engine management system on modern vehicles allows the engine to run on the leanest possible mixture, meaning greater amounts of nitric oxide and nitrogen dioxide are produced.

The more power output the engine gives, the higher NOX gases will be emitted and the greater the need to reduce them. This is achieved by the use of a three-way catalytic converter and the addition of a NOX storage catalyst; this stores the harmful gases and then performs a regeneration – similar to a diesel particulate filter – by altering the fuel calibration. Unlike diesel regeneration, this process is performed around once a minute and controlled by the NOX sensor.