a. The most probable reason is that the module entered Configuration mode. This can happen if the timing of the UART interface is not respected, or data is sent to the module while it is busy, and the module then accidently receives a “0x00” command. To prevent this it is recommended to use the UART handshake signal CTS.
b. It may also be that the Reset pin has been trigged accidently by noise or some other uncontrolled event. Make sure that the Reset pin is connected as shown in the data sheet, using a low impedance driver for the signal.
Last Update: August 8, 2017