'87 240 wagon 245,000 miles - Older AMM - Brand New O2 Sensor - Fresh Oil & Filter

Using 2 meters—One on O2 sensor, other on CO test point (Duty Cycle function) AMM pot set at 350 ohms

While slowly turning AMM pot CW, the CO test point stayed a solid 99.9 to 100%, till finally going O.L. During this time, the O2 sensor (closed loop) showed low/lean reading, then went to nice, fast cycling between .3v and .7v (with the occasional .2 and .8) as AMM pot resistance went up.

After a while I switched the CO meter from Duty cycle to volts and got 14v. Then I shut down and measured AMM Pot resistance, which was at 1068 ohms. I kept cranking, assuming it would start over again (learned it doesn't work like that).

I looks like the sensor and ECU can handle any (rich) AMM pot setting, but why cant I get a signal at the CO Test Point? There's also an '86 in the family with the same "failure".

Any thoughts on this — and should I just set the pot back low, then bring it up to where the sensor voltage starts to look right and leave it there?

My daughter is going back to Martha's Vineyard and the Mass. Emissions test with this car tomorrow. I'm trying to get it prepped as best I can.
(Independent shop rate there is $85/hour!)

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Bruce Young,
'93 940-NA (current)
'80 GLE V8 (Now gone)
'83 Turbo 245
'73 142 (98K)
'71 144 (track modified--and still here)
New 144 from '67 to '78
Used '62 122 from '63 to '67