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[Niall]

So youve got 17s on 3 corners and a 14 on the other? in which case, thats your reason...one wheel is spinning faster than the others

Not necessarily, if the rolling circumference of both the 17's and 14 is the same then there will be no difference. Having said that there is likely to be a small difference but I'd doubt it'd be enough to upset the ABS? More likely to have knocked the sensor when swapping the tyre?

i think you would be shocked as to how sensitive they are! But yes i see what your saying. Even still, i reckon there will be a difference.
The sensors on them are actually really robust...almost a bit too robust if you ever have to replace one. Unless hes trapped a cable, i reckon its difference in wheel size