My 2004 Saab 9.3 was overheating. The shop says I need a new thermostat and a pressure tank. Since i need the car ASAP, I told them to replace the thermostat and I’d replace the tank next week. They say the car will still overheat. The weather isn’t that hot and i won’t be travelling any mountain roads. Why will it still overheat?

Because fluid not under pressure boils out at a lower temperature than fluid under pressure.