2001 Honda Civic mileage: 96,000. Some time ago, I noticed that our Civic was running hot (not overheating yet) and the electric fan didn't turn on. Recently I replaced the thermostat, radiator cap, radiator fan thermo switch but the fan still wouldn't turn on. I know it works because shorting the thermo switch wires turns it on. It normally runs with the temperature parked around center of the scale but in slow traffic will rise to 3/4 scale. Should the fan turn on then? My Haynes manual doesn't show full electrical diagram and I am not sure if only the thermo switch turns the fan on. Some models had relay also.
The radiator fan normally turns on after the temperature is past on the gauge. You may have a faulty thermo-switch. In addition, if you have a blown head gasket it would allow exhaust to get into the cooling system, which in turn would fool the thermo-switch.