V40 fan staying on occasionally.

Engines may have particularly hot spots inside, when the engine is turned off the hot bits can raise the coolant temp. locally, triggering the fan, since there is no coolant circulation (engine stopped) it will take a while before the temp comes down enough to drop the fan out. The fan may also be triggered by the air con detecting high temperature or pressure.

Doesn't sound like anything to worry about.

Outside or inside? It's relatively cold outside, so the missus will have the heating on high, and unless I'm completely misunderstanding it (which is very likely), it seems to combine the A/C with the heating in a way I'm not used to.

I'll do my best to convince her :-)

It could be doing a DPF regeneration. My VW runs the fans flat out when that is happening.

Whatever the reason, it is probably deliberate.

My Pug does it. I presume it does it to cool the engine down.

If it has Bosch engine management, there's a good chance the dash gauge doesn't read true coolant temperature, but goes to the normal position over quite a wide range (but normal) actual coolant temperature. To prevent the driver worrying. ;-)

The cooling fan might well be switched by true coolant temperature, so when towards the upper end allowed, runs the fan after switch off to prevent localised overheating.

That would be more obvious if the car only did this after being stuck in a traffic jam, etc.

Okay, thanks. Something for me to find out about.

I expect so :-) But it's only recently started doing it. And it being Winter, it seems a bit odd.

Good grief. But it does seem suspiciously 'central', come to think of it :-)

I wouldn't give it any thought if it was after a 'busy' drive in Summer; but, if anything, it's the opposite. I'll have to see if anything develops. The missus is all for taking it to a dealer - I'm just trying to pre-empt that :-)

No cooling system ever runs at exactly the same temperature under all normal operating conditions.

I first came across this Bosch trick in a 90s BMW. But you could get the OBC to read true coolant temperature as a check. But of course it is only reading coolant temperature where the sensor is sited - and water isn't a brilliant conductor of heat.

It's a bit like oil pressure. Fit a gauge and people worry about it. ;-)

Idling in heavy traffic usually pushes the temperature the highest - as there is no airflow through the rad, other than from the fan.

This is fairly normal with electric fans (nearly all FWD cars).

When it's switched off with engine up to operational temp the T-stat is open. There is a thermo-syphon circuit from hot head to top of rad, coolant cools as it falls though rad back to engine block inlet. Cooled water pushes hot coolant up to head and round it goes until the T-stat shuts.

As the pump has stopped and there is no forward motion to push cool air though rad, the coolant temp rises and that triggers the electric fan which has it's own thermal switch, usually in one of the rad hoses.

The only way to not have this happen is by driving very gently for the last 2-5 miles.

RWD without electric fans just sit and cook themselves.

and on more than one occasion (personal experience) the system can get so hot that it blows a weak hose, or radiator, or throws some coolant out of the overflow.

Dan S. MacAbre brought next idea :

Heating and A/C on is usually done to try to dry out the moisture in the cabin. Both come in in mine, combined with recirculation sometimes, when I set it on the demist program.

Depends how cold it is. AC won't normally fire up below a set temperature.

The demist is certainly impressive. I'm a bit jealous.

Dave Plowman (News) explained on 03/02/2018 :

The a/c panel is after the heater panel, at least in my car, so it first warms the air, then dehumidifies it - so low temperatures are no an issue for its use. It works a treat, clearing the mist in just seconds.

It happens that Dave Plowman (News) formulated :

Mine does that, it has the opposite effect - it induces worry :-)

Dave Plowman (News) wrote on 02/02/2018 :

You can also get the true temperature on mine, from the OBD, on the screen - but its a bit of a faff to keep doing it. Instead you can buy a small separate display, which reads the temperature from the OBD and shows it constantly.

I expect Torque will show this?

Dan S. MacAbre has brought this to us :

I understand that it will.