Thanks so much. It turns out the sensor is, in fact, a 1mm x 2mm spot at the bottom of the Climate Control Module. Odd place for it, to be sure. It might just need the sludge cleaned out for it to work more reliably. FYI, LEDs can be and are used as light sensing devices as well and it looks like th...

Thanks for the quick reply and good suggestion but it turned out to be something simpler. The electrical connector to the ETM - the one that's so hard to release - was not properly seated. All good now with no relevant error codes.

Follow up a year later.......
I did drive it with the plugged oil return passage for over 1,000km before getting the oil passage cleaned by a shop. No oil blow by and not even one drop of oil on the splash pan after a year.