The ECT and its wiring all the way up to the ECU should be fairly easy to check with an ohmmeter.

With the ignition switched off, unplug the ECU connector and measure with a DMM set to Ohms between pin 13 and ground. Note: to prevent damage to the reasonably fragile contacts, don't measure from the front but take off the sleeve and measure from the sides of the contacts.

If all is well, TP31361-1 section E14 says that the measured value should be:
-10 C/14 F: 8,260-10,560 Ohms
20 C/68 F: 2,280-2,720 Ohms
80 C/176 F: 290-364 Ohms

For those that don't already have that Volvo service manual, it can be found here.

If the ECT turns out to be fine, I'd suggest to start the fault tracing section from the beginning and run through the steps one by one. Every single step should be checked, even if you think you already eliminated a particular item or have the assumption that that item cannot possibly be the cause. Almost all of the steps can be checked with a DMM in either Ohms or Volts measurement setting.

Good luck!