Don't know why the car overheats, please help.

assuming that the coolant level is full, and the system has been bled properly, then it is possible that you have a bad head gasket. Have a combustion leak test done. or if you want to do it yourself, you can buy a test kit for about 40 bucks and there are lots of youtube vids on how to do it.

Be careful, overheating is serious trouble. First of all, check the fluid levels in the cooling system. Does the liquid disappear? if yes, then there is a leak in the system. It might be a simple hose leak or it can be other leaks in the system. If there is liquid in the system, then it does not circulate. Which is another problem. Can you verify the liquid levels.

Thanks for the response. The liquid levels are normal and I've checked all the hoses. I noticed it doesn't circulate and I can't figure out why.

It circulates when the Thermostat opens up. This happens if the engine temperature exceeds optimal operational levels. So if it is hot, and the liquid doesn't circulate then surely it is the thermostat. But you can't check that for yourself as the liquid temperature is fatal when the car is hot. Please don't try to check circulation at that level. The wise thing to do is to remove the thermostat. Get some water and boil it. Then throw the device in it. It should open up if it is work. If it doesn't, then its malfunctioning and you need to change it.

Just realized you already changed the thermostat. I do not know what else might be the problem. did you check the fan wiring?

Thanks for all your response, you've been very helpful. I am now convinced it is the head gasket. Anybody with tips on changing that would be very helpful right now. Could it be as simple as just replacing it or is there anything else I should look out for?

Not sure about all years, but a 2000 s40 / v40 has an air bleeder valve (red circle on hose from radiator on driver side).

Not sure about other years but a 2000 v40 / s40 has an air bleeder valve on the radiator hose on the driver side going into the engine. Looks like a small red plastic thumb screw.