Thank you for the reply.

The O2 sensor heater sees about 13.5 volts when the engine is running. It draws about 2.5 amps when cold and quickly falls to just over 1 amp as the sensor heats up.

O2 sensor signal amperage runs from -3 microamp to +2 microamp when system is running closed loop.

I adjusted the idle mixture (lambda tower screw, mixture control unit trim screw... you know what I mean) so lambda duty cycle at idle was running 75-80%, EHA current was running 8.5 - 9.5 milliamp, car seemed to idle fine, no fault code (obviously... or I wouldn't have been able to read the lambda duty cycle) or CEL. Did not drive car like this, but it seemed to respond okay to lightly applying the throttle in park.

Reading over the manual, I see I'm supposed to aim for 35-45% duty cycle at idle, not 50%:



So I shot for 40% average and as the car sits right now:

idle:
37 - 43% lambda duty cycle
-0.6 to -2.3 milliamp EHA current

and leaving the EHA valve trim screw where I'd last set it, at roughly 2,000 rpm in park with the purge valve vacuum line blocked:
40 - 47% lambda duty cycle
-1.1 to -2.7 milliamp EHA current

So for whatever reason it seems to be running closed loop now, no CEL. I guess we'll drive it for a while and see what happens.