[cheshired5]

The simplest way to check a thermostat nowadays is just to view the coolant temperature with diagnostics. A £5 ELM327 will do just fine for this but you'll need Dice and Vida to recalibrate the CCM or interrogate further for information on the cabin, evaporator and air quality sensor which could all affect the performance of the HVAC system.

I've done loads of D5 coolant changes and it definitely isn't 12 litres despite what the manual says.
Total capacity is 8 litres max which requires 4 litres of Volvo concentrated.
If you put in 6 litres of concentrate, the mixture will be too strong and less efficient at cooling.

After draining the old I would flush the system forwards and backwards with clean water and a final flush with deionised water before draining and filling with 4 litres of concentrate and topping up with deionised water.
The end result will last 5+ years easily and there's a detailed change video on my YouTube channel.