What is an oxy­gen sen­sor and what does it do?

Motor Equipment News - - FRONT PAGE - READER RE­PLY 0140834

An oxy­gen sen­sor mea­sures the amount of oxy­gen in the en­gine’s ex­haust gas. In com­bi­na­tion with the ve­hi­cle’s cat­alytic con­verter, the oxy­gen sen­sor con­trols the ex­haust treat­ment, and, with the en­gine con­trol unit (ECU), the oxy­gen sen­sor en­sures op­ti­mal air-fuel mix­ture com­po­si­tion to en­sure over­all smooth, pow­er­ful, clean and eco­nom­i­cal en­gine per­for­mance.

A ve­hi­cle’s cat­alytic con­verter will only func­tion cor­rectly if the air-fuel mix­ture com­po­si­tion is cor­rect as it is de­pen­dent on the amount of oxy­gen present in the ex­haust. The oxy­gen sen­sor is the cru­cial com­po­nent and feed­back mech­a­nism in the en­gine man­age­ment sys­tem.

Prop­erly func­tion­ing Bosch oxy­gen sen­sors will help: Re­duce fuel con­sump­tion. Avoid ex­pen­sive cat­alytic con­verter fail­ures. Re­duce ex­haust emis­sions. Im­prove driv­ing per­for­mance. There are two main types of oxy­gen sen­sors, “switch­ing” and “wide­band”. A “switch­ing” sen­sor al­lows the ECU to de­ter­mine if the ex­haust is rich or lean and can be used in both pre- cat­a­lyst and post-cat­a­lyst po­si­tions. A “wide­band” sen­sor al­lows the ECU to de­ter­mine how rich or how lean the mix­ture is, and used only in the pre­cat­a­lyst po­si­tion.

The sig­nal from the pre-cat­a­lyst oxy­gen sen­sor is pri­mar­ily re­spon­si­ble for air-fuel mix­ture ad­just­ments, while the post-cat­a­lyst oxy­gen sen­sor sig­nal mon­i­tors the per­for­mance of the cat­alytic con­verter and the amount of oxy­gen be­ing ab­sorbed dur­ing its op­er­a­tion.

Oxy­gen sen­sors have evolved over time, and are avail­able as sim­ple sin­gle and un­heated, two-wire un­heated, three and four-wire heated switch­ing sen­sors and now five-wire wide band sen­sors.

Heated sen­sors al­low the sen­sor to reach op­er­at­ing tem­per­a­tures more quickly pro­vid­ing a quicker re­sponse and more ac­cu­rate data to the ECU, lead­ing to lower emis­sions. Heated sen­sors also have an ex­tended ser­vice life due to lower sen­sor de­te­ri­o­ra­tion from heat stress.

Bosch of­fers a range of di­rect fit oxy­gen sen­sors and a uni­ver­sal range. The di­rect fit type is de­signed to be a

sim­ple “plug-and-play” type sim­i­lar to the orig­i­nal equip­ment item fit­ted on the ve­hi­cle. The uni­ver­sal type sen­sor is an al­ter­na­tive op­tion in cases where a di­rect fit op­tion is not avail­able. The uni­ver­sal sen­sor re­quires some ad­di­tional labour for in­stal­la­tion.

How­ever, in com­bi­na­tion the di­rect fit and uni­ver­sal oxy­gen sen­sor pro­vide a good range to the mar­ket to cover the ma­jor­ity of ap­pli­ca­tions.

When should an oxy­gen sen­sor be checked and or re­placed?

While there are many fac­tors that con­trib­ute to pre­ma­ture oxy­gen sen­sor fail­ure, it is im­por­tant to un­der­stand that an oxy­gen sen­sor is a wear part with a spe­cific ser­vice life not un­like a plat­inum spark plug.

An oxy­gen sen­sor should have a ser­vice life rang­ing from 50,000km to 160,000km, de­pend­ing on its de­sign, but this can be dra­mat­i­cally re­duced by ab­nor­mal op­er­at­ing con­di­tions, in­clud­ing over­heat­ing of the sen­sor, chem­i­cal con­tam­i­na­tion and im­pact dam­age.

Over time, and as the oxy­gen sen­sor de­te­ri­o­rates, the out­put from the sen­sor slows down giv­ing the im­pres­sion the en­gine is run­ning lean with the fuel man­age­ment sys­tem over­com­pen­sat­ing, thus lead­ing to in­creases in fuel con­sump­tion.

It is good prac­tise to check oxy­gen sen­sor func­tion at ev­ery ma­jor ser­vice.

Tips and pre­cau­tions

Bosch oxy­gen sen­sors are sup­plied with spe­cial high tem­per­a­ture re­sis­tant grease for ease of in­stal­la­tion. On fit­ment, oxy­gen sen­sors should be first tight­ened by hand un­til the seal­ing washer is in con­tact with the ex­haust mount­ing boss be­fore us­ing an ap­pro­pri­ate span­ner tight­ened to 40 – 60 Nm.


Wide­band sen­sors must not be dis­con­nected while op­er­at­ing as this can lead to pre­ma­ture sen­sor fail­ure.

The sen­sor heater sup­ply must never be con­nected di­rectly to bat­tery sup­ply. The sen­sors re­quire ECU con­trol of the heater power to avoid ther­mal-shock, es­pe­cially if con­den­sa­tion may be present. Do not ex­pose hot sen­sors to wa­ter as ther­mal shock can dam­age the sen­sor.

Do not drop sen­sors dur­ing in­stal­la­tion as cracks in the sen­sor el­e­ment can oc­cur. Do not use any lu­bri­cants or sealants that con­tain sil­i­cone.

Clean­ing agents, grease, or re­pel­lents must not be used on or near the con­nect­ing plug. The sen­sor must not be ex­posed to ve­hi­cle un­der body sealants, wax, tar or rust pre­ven­tion prod­ucts. Cover sen­sor if un­der­tak­ing body re­pairs.

En­sure the sen­sor ca­ble has free move­ment to al­low for move­ment in the ex­haust sys­tem. For fur­ther in­for­ma­tion on Bosch oxy­gen sen­sors please con­tact 0800 452 896 or visit www.boschau­toparts.co.nz

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