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Great mileage! Almost seems like your car is in trim without the feedback.
The code is being set because the computer doesn't see the quick switches between lean and rich it expects as the sensor reads and the computer compensates. It only sees the offset voltage being provided by the computer itself so it thinks you have no oxygen sensor.
I'm not sure what conditions you're measuring under, but I believe the sensor has to be heated to provide any measurement being so far away from the manifold in the '89, so it is possible the heater circuit isn't getting juice. Best way to check that is to run it for a few minutes, then pull the heater connector and quickly measure the heater resistance. You'll see it falling rapidly from some value above 10 ohms to maybe less than 4-- the actual numbers aren't that important, just the fact the resistance drops significantly tells you it was hot and now is cooling. If the resistance is steady you probably don't have 12V getting to the heater connector or a pin has pushed back.
If the heater is OK and you still get a fairly steady 1/2 volt out of the sensor, it is open. Could be the wire, or the element is broken inside the downpipe. To tell for sure, disconnect the sensor (green wire part) from the computer and check you have that same 1/2 volt coming from the computer but not from the sensor.
The clue that it might take 500 miles to set the light tells me to suspect the heater circuit: a good hard drive might get the sensor going without the heater, so it is just the idling and slow driving that the computer is complaining about.
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Art Benstein near Baltimore
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