Hello All,
I've got some pesky codes popping up and I suspect a bad o2 - but its seems asymptomatic.

O2 Sensor sweeps fine but I have no clue about the rate.
Sensor responds as expected with plugged vacuum line, vacuum leak and raised RPMs.
The only suspicious sign is that on sweeping and in response to plugged vacuum line (might have been the vacuum leak test) the voltage will drop below 0.1v. Generally to 0.09 but I did see a 0.08 once. Does this = fail?

LH 2.4 Codes:
1-2-1
2-2-1
2-3-1

1-2-1 - AMM signal: This is a new rebuilt AMM so I doubt this is the actual cause. I test voltage at the computer and all looks correct.

2-2-1 - Too Lean on part load.

2-3-1 - Adaptive fuel trim too lean or too rich in part-load range (redundant?)

Had cracks in the header pipe (before o2 sensor) for quite a while. I just replaced the pipe with a new aftermarket one because of very poor gas mileage. Upon testing the pipe using soapy water and a Shop vac blowing through the tail-pipe I can see a miniscule leak (but not hear or feel it) at the manifold and at the Cat.

ideas?

Thanks everyone.