Thank you for your responses. The systems you refer me to are for newer cars. Mine is an 83 380SL so I it may be different particularly with KOEO.

Digital meter hooked up as follows: positive lead to #3 term and neg lead to ground (available at term 4). edit:<>

Here is what i got (different than last night):
KOEO: 0Vdc on terminal 3 and ground; 0% duty cycle
Cold idle: batt voltage btwn term 3 and ground; 26% duty cycle
2 min warm up (below 80C): 13% duty cycle
Warmed up and idling at 1200RPM: 26% duty cycle

In my car it seems that with KOEO terminal 3 gives useless info (is this a problem?). BTW, i am using a digital meter.

After the work i did yesterday cleaning the ICV and throttle plate the engine idles too high at 1200RPM and doesn't go back down. That's weird because yesterday after cleaning everything I got the idle perfect, something happened overnight.

With the car running at this high idle and in closed loop I back probed the O2 sensor and got values between 300 and 600mV. I measured the cat temp:
inlet 400C
outlet 400C

Smog test results (done before replacing the defective O2 sensor) showed 3.89% O2 at 15mph and 4.23% at 25mph.

__________________
1972 450SL
1982 300D Turbo