Tackling an electrical issue...is it worth it?

So I got this code MID 128 PID 111 FMI 3 - CUMMINS ISX - which indicates Low level coolant sensor, electrical fault. The truck is not derated or anything like that. Coolant level is good.
The code would disappear for a few hours, even for a day or two and then comes back...
I did replace the sensor - $80 and it did not help. I tested the new and the old with a multimeter for the continuity and they both are displaying the same resistance, which means that the old one was just as good as the new.
I checked the continuity on the harness circuit to the sensor and there was no resistance, the loop was closed...
What am I missing? What's the next step?
The wires run in the bundle which I would hate to unwrap for the fear of compromising other wires, which might be too brittle due to time and wear. Perhaps, it is best to replace the whole engine harness but that's a major operation. It is annoying to see the check engine light....maybe it is better to ignore it...the coolant level is there...so why should I bother?
Knowing myself, I won't ignore it...but kind of afraid to make myself a bigger problem while trying to resolve what seems to be only an annoyance.
There are only 2 wires in the circuit...where specifically do they enter ECM, so it sends me a fault message? I was thinking running a test circuit to ECM directly from the sensor to see, if the problem is with the wires but then why would the continuity test not fail? Could it be the ECM itself? Or maybe I am doing the test the wrong way...I take the multimeter terminals into the plug pins and I read a value of 0 Ohms as opposed to LO, in case of an open circuit?

 

I think it is the best way to do it...
At this stage, I am afraid that I touch one wire and crumble other 3 - more important ..causing more more significant faults.

 

it is volvo isx 2009

 

That's a good question.
I don't have a diagram and I am not sure where the wire ends exactly...