What should happen is that the ECU sets the status to ON when the engine starts, then monitors the output from the sensor. If it doesn't see a proper signal after about 80 seconds it switches the status to OFF. So there were 2 things wrong: The status started at OFF; the sensor never really started working.
However, I had just redone the exhaust join upstream of the sensor and, although the sealant was supposedly sensor-safe, I wasn't convinced it hadn't coated the sensor, so I was hoping that a good run would burn off the containment and then on a second run it would all start working again.
So out for a good run and this was the result:
So it was nice to see that the status started at ON. And also I had been a pessimistic as it seems the contaminant burnt off very quickly and the sensor started to work. What was a surprise was that the ECU left the status at ON for over 2 mins with a non-working sensor. Never seen that before.
Still, nice that everything is working.