What is it and how do I fix it? Fan switch or fan relay?



So yesterday my car overheated. I refilled all of the coolant receptors and the radiator and noticed that even though the temp gauge on my dashboard was above the mid-mark, the radiator fan would not turn on.

Friend helped me, and there is a sort of plug-in type object between the radiator and the radiator fluid reserve bottle. There is energy flowing through the receptor of that object, but not from the plug-in component.

In the meantime, as finances are tight, my friend installed an on-off switch in my dashboard so I can turn on and turn off the fan when I am driving.

Anyone know the name of this object that controls the fan, and how I can get it fixed?


Your friend installed a switch to manually control the radiator fan. Sounds like your car is too “shade-treed” to really know what is going on, or what “objects” are.


how is that helpful? I am trying to determine what is broken. My friend did the best he could, and thinks that I will need to have a computer diagnostic to figure it out. Please offer helpful advice or even questions, rather than unnecessary comments.


Ps. the switch is a temporary fix until I can figure out what the main issue is (I am thinking that something is wrong with the signal or energy from the computer or battery, to the switch that operates the fan).


You did not state the year model of this car but is this object a small square or rectangular box? If so, that could be the fan relay. The relay attains a ground through the radiator fan sensor which closes when the temperature gets high enough.

Picking a year model out of the air, check all of the fuses; especially 9, 45, and 47.
If the fuses are good then the possibility of a faulty radiator sensor should be considered.


thank you. :slight_smile: when you look at a fuse, can you tell if it is not working?

my car is a honda civic dx coupe 2000.


I know when my car is overheating, as it has overheated before, with smoke coming from underneath the hood. Huge amounts of smoke. Thank you.


So, from a honda forum, I was told that it isn’t the fan switch. So is the only other option the fan relay?

Also, my check engine light did not come on both times my car overheated (with smoke coming from the hood). What could be the problem and if the check engine light isn’t working, is that in any way tied to electrical or computer and my airbag safety?

Thank you everyone who has given me advice. I am just trying to figure out what is wrong with my car and how I can afford to fix it. I do not know much about cars, but am committed to learning more. :slight_smile: I think this forum is the best for that.


Yes, a careful visual inspection of a fuse can tell if it’s good or not but a better method is to probe the fuses with a test light or voltmeter.

Wild guessing, I took a stab at the car being an '01 model and looked at a schematic for that year. Whether it’s the same I do not know.
In my opinion, any time an electrical problem exists ALL fuses should be checked because you never know what’s going to be tied in with what. As an example, in the case of the '01 it was shown that the cooling fan circuit was also tied into a circuit that had nothing to do with the cooling fan itself.


Better be careful with overheating a Honda . . . they don’t like it much and cost bi $$$ if you overheat too much/too high. We have a '95 Civic and I just dealt with an episode of overheating. Start simple and check for leaks. Look at the hoses and radiator for seeping coolant. Change the thermostat and radiator cap, re-fill with coolant and drive it . . . watching for the temp to go up. Don’t let it go about 3/4 . . . turn you override switch (the thing you buddy installed) when it starts to go over 3/4. If the fan hasn’t gone on by 3/4 . . . post back. Rocketman


thank you rocketman. :slight_smile: my fan usually turns on right below the mid-mark, so that is what I have been doing with the override switch.

the hoses are fine, refilled radiator and reserve bottle. how do i check the thermostat?


The check engine light should come on every time you go through the starting sequence. Look at your owner manual to see how that works. You might just have a burned-out bulb.

Most people don’t check the function of thermostats, they just replace them. That is because they are usually pretty inexpensive compared to what you would pay someone to just change them. You can check one by submerging it in a saucepan of cool water with a suitable thermometer. Heat the saucepan on a stove raising the temp slowly and note when the thermostat starts to open and when it fully opens. Compare the temperatures to the thermostat rating.

Please be more patient with people answering your post here. They are being very, very patient with you. Your vague descriptions have very little useful information in them compared to most posters with problems. Furthermore, they indicate a fundamental lack of knowledge of practical physics like electricity, heat, smoke, steam,… I think you are overconfident, expecting people to know exactly what you are writing about after an imprecise description, and underestimate the complexity of automobile systems and subsystems. We can work with it, but it will take a lot longer than we are used to. Keep your expectations for the rate of resolution of this problem very low. Keep going, however, we have some progress already.


I am sorry if my responses seem inpatient. I have received kind replies from people asking me for clearer explanation–to which I respond in kind. I have received a few snarky comments that have not been very helpful. I do think that asking or suggesting that someone doesn’t know that their car is overheating or belowing smoke is a bit snarky, yes? I do not think that I am overconfident, nor do I lack basic knowlege of certain things.

I am trying to ask the question in the best way that I know how. I am trying to get an idea of the problem and so far I have gotten a good amount of info. I am very grateful for those that have taken the time to assist me with my questions. I do really appreciate all of their effort and hard work.

I looked at my first question, and I do admit that I needed to be clearer by stating the make and model of my car, etc. Next time I will be more clearer in my requests and ignore those who may be snarky.:slight_smile:


how is that helpful?

It is about as helpful as can be. Once someone starts modifying a car in unconventional ways, it really makes trouble shooting difficult and even more so when doing it long distance.


The temperature gauge was all the way past the red mark as far as it could go. I checked under the hood to see what was going on, but did not smell burnt coolant, or spillage of coolant.