Kausi

I'm tracking down a rough idle issue and looking on XENTRY at O2 sensors. The upstream sensors seam to work fine (both fluctuate normally ca 80 to 730mV). Right downstream seems to work normally (at my limited knowledge) as well. Steady at 690mV. But left downstream is steady around 100mV or below. What could be the reason for so big difference? Faulty sensor, cat issue? How to troubleshoot it further?

GinDistiller

I would venture a guess that it's more likely a sensor problem, in the past I've seen bad cats cause a fluctuating o2 reading on the downstream sensor that mimics the upstream. It sounds like your upstream sensors are working properly, and the downstream are just used for emissions so will not affect your idle. I'd start looking at other sources for the idle concern. Any codes in STAR? Fuel trims ok? How old are the engine mounts?

kevm14

Could also be an exhaust leak upstream somewhere letting fresh air into the exhaust. Fresh air will pull down the O2 readings.

teknic

I'm guessing you have not gotten any error lights, but I would look closely to the O2 sensor. Mine started with a rough idling too first and then started throwing a code for the upstream sensor. Replaced it and is back to normai now