the system works because petrol nozzles are 21mm and diesel nozzles are 24mm (this obviously stops you filling a petrol car with diesel as the larger nozzle won't physically fit in the hole).
BMW's now have a valve in the filler neck that requires 2 latches, one on each side of the filler, to be pushed in. The correct 24mm nozzle is wide enough to push in both latches at the same time, the 21mm nozzle can only ever reach one latch at a time so won't open the valve.
To a point I blame the garages on this continuing problem, they should never put the derv pumps in the same LINE as petrol pumps, it occurs all the time & people will make costly mistakes, but at the end on the day it takes you two Min's to check & recheck that the pump you have stopped at is Petrol --- Or derv whichever product you need, to your Question yes there is a difference in the nozzel between petrol & derv.
kira000
even with the devices now fiited on Fords i still know of some plank who managed to get it wrong, an agency delivery driver.
Went to fill a mondeo up and couldnt get the unleaded nozzle in ( it was a diesel vehicle) In the tool kit of the Mondeo there is a funnel for emergency use,ie filling from a can.
He managed to use this with the nozzle and fill up with petrol, caused approx �350. worth of labour to put it right.....nothing is foolproof !!