Part of my Chevrolet stable is an '04 Cavalier that has been a real work horse. Almost 100K and no issues. Now, however, it appears that the cooling fan is not coming on. I am told that it should come on when I put the system in high defrost (which activates the AC). NADA. Fuses are good, but I have no understanding of the designated relays — how to test, etc. Any experienced insight would be very much appreciated, as I can repair most of this stuff if I have an idea as to what's wrong.

Work your way backwards. Put the ac on high, pull the connection at the fan and check for positive and negitive. If you have both, you have a bad fan. If you don't have one or the other, then start checking relays and such.

the 2.2 cavaliers are known for 3 things fan problems, fuel pump problems, and warped heads and bend rods due to blown head gaskets. do a search on google for overheating 2.2 cavalier. 20+ pages come up. first check the main fan fuse and relay. the one i worked on the fan motor actually burnt up. unplug the connector at the fan and use some test leads and supply the fan with a direct power and ground to see if its good or not. if not you can get just the motor at napa for around $40. if the fan, fuse, and relay are good its the ECT sensor that monitors the colant temp and tells the fan when to kick on. to diagnose it just remover the connector, if the fan doesnt come on the sensor is bad ($10) if your standing in front of the car looking at the motor its on the hard coolant line on the left front of the motor its just a 2 wire sensor that threads into a bung on the coolant line, if it is still not working after that then the only other thing i can be is a fauly ecm which is very rare. also these motors are very picky about air in the lines and they are very prone to warping the head when hot so be sure to bleed the coolant system with the small bleeder valve on the coolant line on the right side of the motor and be carefull it is very easy to snap off. the one i worked on had already be hot so many time the head was already warped so i removed the thermostat to keep it cool and give it a few more weeks of life, the keept driving it hot so now its got a blown gasket and bent rods. good luck and keep this thing cool or you will start having head problems quick