Your duty cyle at those two pins should fluctuate. This is indicating you are not achieving a closed-loop system. As you said as well, the fact that deflecting the plate does not change it means there is a problem.
You don't need a frequency DMM for this. Voltage is good enough. 13.6v = 100% rich, 0v = 100% lean. You want a fluctuating voltage around ~6.5v
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Originally Posted by landiss.com
Assuming the "standard" 49-state controller (70% duty cycle with ignition on but engine not running, or about 4 volts average), the following tests should be performed before attempting to adjust the idle mixture.
1) Idle Contact: Deflect the air flow sensor plate. The duty cycle should decrease to 10%. If it remains at 70% test the throttle valve switch (Job 07.3-121).
2) Full Load Contact: Open the throttle completely. The duty cycle should decrease to 20%. If it only decreases to 40% test the air flow sensor potentiometer (Job 07.3-121).
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Try the above and report back.
Also, pull the error codes because it might be something the system already has stored.
Of course, your cat could be suspect, but with your system not going into closed-loop, this should be something that needs to be addressed regardless.