Hi, I drive a 2000 Subaru Legacy and the radiator fan does not always come on. This sometimes makes the car run hot but has not (yet) put it into the “danger zone” overheating. It seems to happen if the car is in stop and go traffic and it’s hot outside. If I’m driving down the highway or the weather is cool it doesn’t happen.
I checked the fuses (yes, even the ones under the hood) and they are all fine. I am just curious if there is a sensor that could be the problem or if it is more likely the thermostat. It didn’t quite fit the descriptions I found for thermostat but I’m a broke college student trying to keep an old car on the road, not a Subaru mechanic 
If your not handy, and don’t have the equipment necessary to determine the obvious electrical problem, then you need to see a mechanic that has the equipment and knowledge.
The problem must be narrowed down. It could be a bad cooling fan. It could be a bad relay. It could be a bad sensor. It could be a bad connection or bad wiring. The thermostat is not the problem. A proper diagnosis can prevent replacing parts that are not bad and could very well save you money.
If you do let it sit at idle will the fan eventually come on, but only after it gets hotter than normal?
Mostly BustedKnuckles is probably right since there are plenty of things involved. But there is one thing you might be able to figure out. Often cooling fans have 2 speeds - hi/lo (I don’t know the specifics on your car). There is often a relay for each speed - 1 low speed relay; 1 high speed relay.
Its also the case that many of the different relays for different functions are actually the same parts. So if you can figure out if you have low/hi relays, you could find another identical one in your relay box that serves a non-essential function that you know is working (e.g. A/C). Swap one in & out for the fan relays & see if that gets the fan running.
Like I said, I don’t know the specifics on your car but its worth a few minutes to try to figure it out.
~ Ok - if you get the “fatal error” message, don’t back up and try to post again. Apparently it posts anyway despite the “fatal error”
- so this was a dup of the above.
I’m handy and becoming skilled (Haynes guide = bible in my household) but I definitely don’t have the equipment to diagnose and treat an electrical problem. My husband might as he’s a HVAC tech and has a van full of tools…
The problem is intermittent so most times the fans come on but when they don’t is when it gets hot. I think I may have to bring it back to the mechanic up the street and see if they can get it overheating. The kicker is, it’s been cool and the fans have been fine. When it is hot outside and I’m in heavy traffic is when they malfunction.
Everything else works, a/c, heat, fans, etc. I do need to change the IAC on the Subie, would that affect the radiator fans? Shot in the dark but the IAC is making it idle hard…
There should be a fuse and relay panel for the fans just to the left of the Main Panel for the fuses under the hood. Remove the cover and check the fuses and relays inside it for a problem.
The ECU controls the relays for the fans. From what you say about the intermittent operation I would suspect a faulty relay or bad connection to the fan as the trouble. You don’t need anything more than a simple test light probe to check power with to see if the relays and fuses are ok and getting power.
IF you have a Haynes & an HVAC tech husband I’d be really surprised if the two of you couldn’t figure this out. The hard part is if it is something weird - like an intermittent problem (e.g. heat sensitive relay) - but that’s hard for anyone.
Personally, I’d swap the fan relay with another of the same type & see what happens.
Usually this is a bad sensor. I’m not familiar w/Subaru, but with many cars the sensor screws in somewhere near the top of the engine, and a wire usually runs from the sensor to the fan motor. The sensor is a gadget that is simply a switch. When the temperature reaches a high enough temperture, the switch connects, and this turns the fan on. I’ve had this fail before on a Toyota, and the way I discovered the problem was to unscrew the sensor, and test the electrical continuity when it was cold, and when it was hot. To get it hot, I place it in a pan of water and heated the water on the stove, with a thermometer in the water. I think it is supposed to turn on (the switch closes) at about 175 degrees or so.
The second most common cause for this is that the wire comes loose from the connector, usually near the fan, caused by someone working on the car in that area and not being careful.
The fan motor can fail too. Usually when it fails, it doesn’t work at all. Since your’s in intermittant, this probably isn’t the problem. Cross your fingers, as the fan motor can be expensive to replace.