If you are looking at current, here are some things to look for:
Closed loop operation will be a changing current going up and down about 2-3ma. it should be centered about 0.0ma but can occur anywhere between -12ma and +12ma. In other words a ranging from +5ma to 8ma and back and forth is still closed loop. It will represent a rich correction (positive current is rich correction) of a lean mixture as determined by the O2 sensor.
Key on engine off should look like +20ma in most cars. Starting enrichment will be over 50ma and cold running will get up to 20ma correction. While running you should not see corrections above +12ma when warm. Lambda correction stops at +12.
Whatever range of correction you find yourself when in closed loop, you should see basically the same range at any steady rpm. In other words if you adjust it like I like: to a range of -3ma to 0,0ma at idle, it should be within ten percent of that at 2000rpms.
Bosch Master, ASE Master, L1
33 years MB technician