NOX sen­sor

Car Mechanics (UK) - - Help! -

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.

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