What do the LED lights on the top of the Reader indicate? There are 4 LEDs on the top of the Reader used to indicate progress of a transaction. Blue – For contactless transactions, indicates that the Reader is waiting for card. The Reader will also beep, indicating you should swipe or tap the card. Yellow – Indicates the Reader is reading the card. Green – Indicates the card read was successful. Red – Indicates a card processing error. Note: The yellow and green lights turn on and off quickly. For additional support, please refer to the Support Page in your merchant portal or contact your ISO contact.