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The front o2 sensor is for regulation and the rear o2 sensor is for monitoring. OBD2 vehicles are required to monitor cat efficiency.

So, the lean condition is being detected by the front o2 sensor.

Unless you are dropping codes related to cat efficiency (P0420) then it is my opinion that replacing the rear o2 sensor is not worthwhile.


The rear O2 sensor is often used for 'fuel trim.' Cannot speak for the T100 but on many vehicles the rear sensor does play into A/F ratio in addition to its primary purpose as the catalyst diagnostic. The front sensor usually degrades sooner so the rear sensor corrects for 'drift' in the primary [front] sensor signal.

It is unlikely though that the rear sensor would go bad without triggering a cat code or a dedicated rear sensor code. 'Lean' fuel codes are usually just that - the engine is running lean. You could pull the plugs and look for color differences to see if it is injector/cylinder specific.

Jeff
96 T100 SR5 4WD Ext Cab, Warn Hubs, HD torsion bars, Bilsteins, Camper Shell, TJM bar, factory 31 x 10.5's, Powertrax No-slip rear, Recaro driver's bucket