Reasons: 1.GPRS module doesn’t install in the right place.  2.GPRS module fault.  3.GPRS interface fault.

Solution steps:  1.Unplug GPRS module, install it again to ensure it is in the right place.  2.change GPRS module.  3.If it still useless,

It may be that the GPRS interface of the concentrator is damaged and the concentrator should be replaced..