Be sure to check the fan relay in the fuse/relay panel under the hood. A friend has a 1998 Monte Carlo and we replaced the thermostat twice before realizing it was an electrical problem.

If you find it isn't the temperature switch, relay, fan, or connections - I have bad news for you. You may have to replace the computer. This sounds drastic, but it is true, at least if you want to pass an emissions test. A few years back, the fan relay was turned on and off by the temperature switch. It had one green wire coming off it and when the temp reached a certain level, it closed and turned on the relay and thus the fan. Later, GM decided to let the computer control the relay. The temperature switch only sent a signal to the computer and the computer decided if it was 'right' to turn the fan on or off depending on other factors. The bad news is, the input on the computer has sensitive transistors that have been known to burn out. Consider it a weak point in the design. This doesn't mean that the computer will fail to make the engine operate. It only means that it is incapable of turning the fan on and off or sometimes read the engine temperature which will make the fuel mix wrong at temperate times of the year. If you really don't feel like spending the $$ on a computer, the relay can be hotwired. Simply cut the green wire going to the relay and ground it. The fan will run any time the ignition is on. This may be a band-aid fix, but it sure beats having your car overheat when sitting in traffic - at least till you splurge for a computer.