In most cases, the cause of an overheating or no-heat condition in your vehicle is a faulty thermostat. And since a new “T-stat” costs only about $8, it makes more sense to replace it than to spend hours diagnosing the problem. If that doesn't fix it, at least you're only out about two hours.
Pick up a new T-stat and gasket, as well as RTV sealant, fresh coolant (to top off the system) and hose-clamping pliers at an auto parts store. And while you're there, ask the clerk for the torque specs for the gooseneck bolts. Then gather up your metric sockets, a plastic scraper and a drip pan. Slide the drip pan under the engine to catch the spilled coolant.
The T-stat is usually located near the top of the engine under a “gooseneck” housing attached to the upper radiator hose. If yours isn't there, consult a shop manual to locate it. Remove the two or three bolts that hold the gooseneck in place and remove the T-stat (Photo 1). Next, clean both the engine and the gooseneck sealing surfaces (Photo 2). If the parts store gave you a plain gasket, coat one side with RTV sealant (self-adhesive gaskets don't need sealant). Then install the T-stat and gasket (Photo 3). If the old T-stat used a rubber O-ring instead of a gasket, lubricate the new one with fresh coolant before you insert it. Reinstall the gooseneck and top off the coolant.