I wish it was as easy to fix this on the 740 as on the 240. I only found the one sensor in the head under the intake, it has a two wire plug. The wires are good and the connector is tight. The gauge NEVER moves, not on start up or while driving. Are there quick fixes like on the 240? Should I find a used cluster and swap it? The sensor looks new but I haven't tested resistance.

 

try whacking the dashboard above gage and see if the gage comes alive. (Percussive troubleshooting) Like any electrical circuit you need power (you can measure the two wires to the sender) voltage should change as car warms up, second thing you need is load device (in this case a working sender andgage) and a ground return. IfANY of the three is missing the gage will not work. a wiring diagram is a great help here and a usable digital volt ohm meter can be had for as little as $4 at harbour freight. For alot more insight than I can give I recommend Tracy Martins "How to diagnose and repair automobile electrical systems" Volvo makes very good factory wiring diagrams but I do not have one for your car. I will try to post link.

https://www.volvotechinfo.com/index....53EF97EDDE88EA

 

The Gauge Temperature sensor has two wires:

yellow/white= goes to gauge
yellow/black= goes to ground

Check the grounding on the yellow/black wire for ground continuity(both directions) & fix if there's no ground,then check the gauge by jumping both wires together if you did that & the gauge didn't move, then you've narrowed it down to the gauge. If the gauge does work then it's probably the compensation board on the back of the instrument panel.
Or like the above thread stated it might just be stuck!
Hope this helps!

*Note* Don't confuse the Gauge Temperature Sensor with the Engine Coolant Sensor which also has two wires but one wire on the ECS goes to the fuel injection computer & the other wire goes to the ignition computer.
The Pic below is of the Gauge Temperature Sensor location(i took out the GTS and installed and aftermarket gauge sender because the aftermarket gauge is more detailed & MY gauge stopped working because of the board)