On 26/08/2014 at 08:33, Pingirons2013 said:

Had problem with fan starting up when car cold or warm . Did not matter if ignition was on or off . Easy fix. £13 . Locate external temp sensor. Replaced with new one and problem sorted. If this sensor is damaged or faulty the car assumes that the engine is too hot especially if it's a warm day or night. The management system cannot trace an external temp so assumes the worst scenario and engages the fan continuously. Hope this is helpful as it works for me. Have now fixed several octavia and passats since

Can you show me where the External Sensor is please, maybe with a picture?

On 24/02/2010 at 21:41, fbrj said:

Eu tive exatamente o mesmo problema, mas com Octavia Mark 1 (2003). Não tenho certeza se a correção será a mesma, mas acabou sendo um interruptor de relé (localizado sob o capô e não no carro). Mudou o revezamento e ficou perfeitamente ok.

Sorry you're talking about something old, but remember which one?

My 2010 Scout 2.0 diesel has got this problem now. Battery flattened, replaced the coolant temp switch and the battery but the fan is still coming on randomly with the engine off. Can't walk away and trust it not to come on while I'm sat oblivious in the house so I'm having to disconnect the new batt all the time to avoid another flattening! Went through the whole engine bay fuse box, all looks OK in there, gave the fusebox area a good blowover with the hair dryer because this has been a cause of other electrical issues in the past (multiple bulb malfunctioning) but it was actually dry and all the fuses looked good. Hoping it's the chafed wiring that has been mentioned on here rather than the whole fan replacement thing. Going in on Friday for my local guy to hopefully trace the issue. 

OK, a result! And a really easy one, now we know! After a good visual check (wiring all perfect) and an exhaustive Snap-On OBD diagnosis we discovered nothing wrong other than a load of fault codes probably caused by me disconnecting and reconnecting the new battery for the last few days. I hadn't trusted the car not to flatten my new £80 battery soI had been disconnecting it after every trip. Arriving at the garage I left it connected and noticed that the fan wasn't coming on. To cut a long story short we reckon it was simply the old and presumably failing battery causing the fan run-on. It hasn't happened since so we're thinking that's what it was. Shame I wasted money on the coolant temp switch but at least I know I've got a new (genuine) part in there now. So, very much worth checking the battery condition if you have this fan problem. 

@Blackpuddin

 

I believe a tired battery is the root of many electrical gremlins.

 

The theory being, when cranking the engine, the battery doesn't have enough in reserve to keep the electronics properly supplied.

 

This power brownout can corrupt some of the data giving problems like the one you've experienced, necessitating a reboot if you like by disconnecting the battery, turning ignition and hazard lights on to completely discharge the system,  then refitting a new or recharged battery to prevent reoccurrence of problems.