A banknote with a different thickness has been detected, or 2 banknotes may be stuck together (“double note detection”) 
 
When thickness counting errors occur frequently, or when banknotes are not being fed smoothly through the machine, it may be necessary to adjust the hopper adjustment wheel (can be found at the back of the device). This wheel controls the thickness distance of the banknotes. The position of the hopper wheel has already been set in optimal position in the factory. However, due to the nature of some banknotes an adjustment may be required to ensure optimal feeding. 

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