Not the usual CPU fan error

i'll start with spec:

i7-3770k
sabertooth z77
artic freezer 7 pro rev 2
8gb corsair vengance black
evga gtx 680
120gb intel 330 ssd
500gb hdd
corsair 850w enthusiast psu

problem is:

when I boot I get an error saying

'CPU Fan Error! Press f1 to setup..'

I've looked around at other threads about this and I can't find one that has the same details as mine, reason being people are saying there fan speeds, although being flagged, are constant. The difference i'm having is that in my BIOS the fan speed is shown as constantly changing between about 600rpm and 19000rpm without any actual difference to the fan (sound + speed). I tried plugging in the standard cpu cooler and didn't get any errors so i'm wondering if my problem is fan related rather than the other threads which suggest BIOS updates etc.

The point of my thread is to save me a trip to the retailer to ask for a replacement fan if anyone can give me info regarding fans? Do aftermarket fans react differently to standard fans? could this be the reason for me not getting the error when booting with the standard cpu cooler? If this is the case I can't start to rule oyut the problem being the aftermarket cooler.

I'm sorry that this may be hard to understand, i'm terrible at wording things.. Thanks for reading!