Tyre pressure diagnostics.

Funny thing happened today after rotating front wheels to back each seperate side the tyre pressure warning light flagged up. Got a mate with a diagnostic fault finder to check it out wich showed the fault up.
I did not realise that all you do is switch the ignition on then press and hold the button for the tyre pressure monitor on the centre console till you hear an audible signal.
HEY PRESTO.DONE.Now just need to get the mpg clock sorted.Stuck around 42 now.Doesn't seem to want to go up to 47 48 mpg like it did a few weeks back.Anyone had this problem?

I altered the tyre pressures after swapping them over. Quite right though mines 32 front and 29 back.

Swapped wheels back now. Reckon on only needing to change one pair of tyres instead of all four when its time soon.Recommended not swapping them over at all from the begining but will put new ones on the rear when replaced fronts.