Downstream O2 Sensors

[jcaspar1]

Having trouble with smog and it seems one bank is running rich. Took it to the dealership and they flushed the injectors and told me the rear O2 sensors were reading funny. Said I should replace them as this could be part of the problem. My understanding is the rear O2 sensors were only to monitor the cats and had no effect on mixture. With no check engine light, is there any reason to change the rear O2 sensors??

 

[Allan]

You are correct about the rear O2 function. They have no effect on drivability or fuel trim.
With no codes, that means they are passing the PCM's catalyst monitor test while driving, so don't change the sensors.

 

[Westxsrt10]

What year? ...and what mods do you have?

You can have hidded INC. codes that show up on Calif. smog monitors. (drive cycles may be needed to clear them) LINK The relationship between the front and rear sensors can cause a failed test.