All the info I can find online says to check resistance at the ECU by removing the cover and probing pin 5 (ground) and 13 (temp sensor). I have the ECU removed but could not find any ways to "remove" the cover to probe the pins. Instructions say it must remain plugged in so I'm either looking at the wrong connector or need some help. I could not find any photos or videos of someone actively probing. Thanks in advance.

First post with all original 346k miles

 

I think the intent is to unplug the ECU from the long connector to find the pins to the sensor wiring. If you are trying to test the coolant temp sensor - plan B would be to do it from the sensor itself or simply remove the sensor and test out of the car (or just replace it if suspect). Not sure about your model, but on my 850 the ECT was right under the thermostat housing where the big radiator hose goes to the top of the block. Its a good idea to service the thermostat periodically so I'd just replace both the ECT and t-stat at the same time if they are old. You can google for a resistance chart but you can use the 300/3000 rule - 300 ohms (give or take ) at operating temp, 3000 ohms at room temp (colder = higher impedance).