Clean your MAF sensor.  Often O2 codes are erroneous and caused by a dirty MAF.  If it ever had a K&N or other oil soaked filter on ti the MAF tends to pick up that oil over time and it throws the reading off. 

Also, check your spark plugs.  If one is fouled it can also cause a rich code.

Verify that the heater in the O2 is getting power. 

Remember that factory O2s are not wideband, so "rich" and "lean" are read only in a very narrow margin off stoich.  If its close to stoich, but off a tiny bit you would never notice that it seems rich or lean. 

1st, its generic advice because I've had O2 sensor codes thrown on several vehicles due to an issue with the MAF.

2nd, given the low tolerance of the O2 sensors its hard to say if one is not reading the slightly rich mixture that is all it would take to trigger a rich code. 

I don't know how much stock I place in a cylinder balance test on these systems.  They have short and long term fuel trims in the programming, but not on a per cylinder basis.  These systems are very simple compared to the diagnostics available in modern EFI systems.  The EECIV can't for example pick up a cylinder misfire.  Really this test is designed to catch an injector that is no performing to a degree that deactivating it makes little to no difference.  You would notice something that far off driving the car.  They run decent as a V7, but not that decent.