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Why should I replace sensors in pairs?

At O2Sensors.com.au we strongly recommend changing sensors as pairs. That is BOTH PRE-Cat or BOTH Post-Cat sensors at the same time.

Sensors that are not performing at maximum efficiency may not cause the check engine light to illuminate, but will usually cause the air fuel ratio to move to rich which can effect fuel economy and cause possible damage to the catalytic converter.

A slower sensor may be operating at some level; however this may cause an imbalance or loss of synergy in the engine management system.

As can be seen in the short 5 second scan for a VS Commodore V6 below the top blue track (Left bank oxygen sensor) is operating slower than the lower red track (right bank oxygen sensor). Although the difference appears small, this is a huge difference if we consider it over 1 hour of driving.

Example of different sensor performance on a VS Commodore V6

vs commodore example oxygen sensors