I have a 2004 Chevrolet Optra and my cooling fan doesn't start. I plugged the fan directly on the battery and my fan worked. The fuses and relays in the fuse-box are fine. My cable seem fine. My temperature sensor seems fine, I was told to unplugged it to see if my fan start and it didn't. I was told it can be a switch relay direct on the fan but it doesn't seem to have one. The car has a single fan, no AC. What can be the problem?

Update: My friend's father is a mechanic and he found the problem: There is a cable missing on a temperature sensor. I didn't know I have two (one for the fan and one for the gauge in the dashboard). It's the cable with the kind of clip on the image below. temp sensor

But now, I can't find the cable under my hood and I don't know the name of this cable. Anyone knows?

  • Welcome to the site. How did you verify the relay as working? Commented Apr 15, 2017 at 22:54
  • I switch it with another relay and the fan didn't started but the other relay worked. Commented Apr 16, 2017 at 15:17

2 Answers 2


If everything you're saying is true, (I kinda doubt it, but I'll roll with it) check your switch source is sending a high-enough current. If the cable to the relay is slightly damaged or if the resistance is too high due to corrosion, it will not switch the relay on. Figure out from the switch, work your way down. Switch, relay, fuse, fusebox, temp sensor, fan solenoid (if one's present.) etc.

  • I can't find the switch or what is supposed to looks like. How can I find it? Commented Apr 16, 2017 at 15:21
  • Your switch is basically whatever tells the relay for the fan to turn on. The relay receives power, but is also a switch that's triggered by another switch, but running at a lower amperage and voltage. The relay 'switch' wire (also called exciter cable by some) should be powered by either the ECU telling it to turn the fan on when the engine temperature rises, or by the temperature sensor itself. Though, some vehicles have more complex systems.
    – yollooool
    Commented Apr 17, 2017 at 1:38
  • Is there a way to know what exactly is turning the relay on? Commented May 17, 2017 at 13:11
  • Apologies for a late response; Usually, you can find the wires attached to the relay, and remove them from the relay, two at a time. Hook them up to a test probe and flick the switch you expect to work. Theoretically, this should work, but check the wiring diagrams for which wires would be the trigger.
    – yollooool
    Commented Aug 31, 2017 at 20:53

Check again all your relays and your cables which are pluged to the alternater/battery. Check your circuit breakers too!


Try to replace the relay! Here is a video about a man who had the same problem as you, it is nearly the same car.: https://www.youtube.com/watch?v=W2xW4dDugdc

You may want to replace the the fan plug as well. Symptoms were: Running hot at idle, engine surging as the AC compressor cycled off and on (blew warm when this happened) due to the cooling fan not working, no trouble codes tripped. You can replace just the fan motor from autozone for about $25.

The coolant temperature sensor will trip a trouble code if it has failed. When it goes out it mimics a blown head gasket with constant overheating.

  • All checked and with someone who know this better than me, he think the problem is the thing that activate the relay, but we can't find it. Commented May 17, 2017 at 13:06

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