How to Fix Modem Manager is not Working on CentOS 7 . Modem Manager is a service that provides support for USB Modems to run on all major Linux Distributions. If Modem mangers is fail to run the services you modem does not work any more it provides the support for USB Dongles and modems. CentOS 7 has bug that the modem manager is services are not started in systemd system and service managers.

Pre Steps

Step-I (Check whether USB Modem / Dongle is detected by the Linux Kernel)
Plug-in your USB Modem after that type the following command on Linux Terminal
root@techbrown ~ # dmesg | grep -i usb
Sample Output
[ 0.208173] ACPI: bus type USB registered
[ 0.208173] usbcore: registered new interface driver usbfs
[ 0.208173] usbcore: registered new interface driver hub
[ 0.208173] usbcore: registered new device driver usb
[ 0.686323] ehci_hcd: USB 2.0 'Enhanced' Host Controller (EHCI) Driver
[ 0.686627] ehci-pci 0000:00:1a.7: new USB bus registered, assigned bus number 1
[ 0.696244] ehci-pci 0000:00:1a.7: USB 2.0 started, EHCI 1.00
[ 0.696345] usb usb1: New USB device found, idVendor=1d6b, idProduct=0002
[ 0.696351] usb usb1: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.696355] usb usb1: Product: EHCI Host Controller
[ 0.696359] usb usb1: Manufacturer: Linux 4.1.5-200.fc22.x86_64 ehci_hcd
[ 0.696363] usb usb1: SerialNumber: 0000:00:1a.7
[ 0.696553] hub 1-0:1.0: USB hub found
[ 0.697147] ehci-pci 0000:00:1d.7: new USB bus registered, assigned bus number 2
[ 0.707075] ehci-pci 0000:00:1d.7: USB 2.0 started, EHCI 1.00
[ 0.707171] usb usb2: New USB device found, idVendor=1d6b, idProduct=0002
[ 0.707176] usb usb2: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.707180] usb usb2: Product: EHCI Host Controller
[ 0.707184] usb usb2: Manufacturer: Linux 4.1.5-200.fc22.x86_64 ehci_hcd
[ 0.707189] usb usb2: SerialNumber: 0000:00:1d.7
[ 0.707363] hub 2-0:1.0: USB hub found
[ 0.707656] ohci_hcd: USB 1.1 'Open' Host Controller (OHCI) Driver
[ 0.707683] uhci_hcd: USB Universal Host Controller Interface driver
[ 0.707951] uhci_hcd 0000:00:1a.0: new USB bus registered, assigned bus number 3
[ 0.708053] usb usb3: New USB device found, idVendor=1d6b, idProduct=0001
[ 0.708056] usb usb3: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.708059] usb usb3: Product: UHCI Host Controller
[ 0.708061] usb usb3: Manufacturer: Linux 4.1.5-200.fc22.x86_64 uhci_hcd
[ 0.708063] usb usb3: SerialNumber: 0000:00:1a.0
[ 0.708229] hub 3-0:1.0: USB hub found
[ 0.708624] uhci_hcd 0000:00:1a.1: new USB bus registered, assigned bus number 4
[ 0.708724] usb usb4: New USB device found, idVendor=1d6b, idProduct=0001
[ 0.708727] usb usb4: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.708729] usb usb4: Product: UHCI Host Controller
[ 0.708732] usb usb4: Manufacturer: Linux 4.1.5-200.fc22.x86_64 uhci_hcd
[ 0.708734] usb usb4: SerialNumber: 0000:00:1a.1
[ 0.708896] hub 4-0:1.0: USB hub found
[ 0.709301] uhci_hcd 0000:00:1a.2: new USB bus registered, assigned bus number 5
[ 0.709389] usb usb5: New USB device found, idVendor=1d6b, idProduct=0001
[ 0.709392] usb usb5: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.709395] usb usb5: Product: UHCI Host Controller
[ 0.709397] usb usb5: Manufacturer: Linux 4.1.5-200.fc22.x86_64 uhci_hcd
[ 0.709400] usb usb5: SerialNumber: 0000:00:1a.2
[ 0.709560] hub 5-0:1.0: USB hub found
[ 0.709929] uhci_hcd 0000:00:1d.0: new USB bus registered, assigned bus number 6
[ 0.710032] usb usb6: New USB device found, idVendor=1d6b, idProduct=0001
[ 0.710035] usb usb6: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.710038] usb usb6: Product: UHCI Host Controller
[ 0.710040] usb usb6: Manufacturer: Linux 4.1.5-200.fc22.x86_64 uhci_hcd
[ 0.710043] usb usb6: SerialNumber: 0000:00:1d.0
[ 0.710199] hub 6-0:1.0: USB hub found
[ 0.710571] uhci_hcd 0000:00:1d.1: new USB bus registered, assigned bus number 7
[ 0.710659] usb usb7: New USB device found, idVendor=1d6b, idProduct=0001
[ 0.710662] usb usb7: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.710665] usb usb7: Product: UHCI Host Controller
[ 0.710667] usb usb7: Manufacturer: Linux 4.1.5-200.fc22.x86_64 uhci_hcd
[ 0.710669] usb usb7: SerialNumber: 0000:00:1d.1
[ 0.710825] hub 7-0:1.0: USB hub found
[ 0.711213] uhci_hcd 0000:00:1d.2: new USB bus registered, assigned bus number 8
[ 0.711305] usb usb8: New USB device found, idVendor=1d6b, idProduct=0001
[ 0.711308] usb usb8: New USB device strings: Mfr=3, Product=2, SerialNumber=1
[ 0.711311] usb usb8: Product: UHCI Host Controller
[ 0.711313] usb usb8: Manufacturer: Linux 4.1.5-200.fc22.x86_64 uhci_hcd
[ 0.711315] usb usb8: SerialNumber: 0000:00:1d.2
[ 0.711478] hub 8-0:1.0: USB hub found
[ 0.711698] usbcore: registered new interface driver usbserial
[ 0.711710] usbcore: registered new interface driver usbserial_generic
[ 0.711720] usbserial: USB Serial support registered for generic
[ 0.744601] usbcore: registered new interface driver usbhid
[ 0.744603] usbhid: USB HID core driver
[ 1.021247] usb 1-5: new high-speed USB device number 2 using ehci-pci
[ 1.144044] usb 6-1: new low-speed USB device number 2 using uhci_hcd
[ 1.306290] usb 6-1: New USB device found, idVendor=046d, idProduct=c05a
[ 1.306296] usb 6-1: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 1.306301] usb 6-1: Product: USB Optical Mouse
[ 1.306305] usb 6-1: Manufacturer: Logitech
[ 1.322872] input: Logitech USB Optical Mouse as /devices/pci0000:00/0000:00:1d.0/usb6/6-1/6-1:1.0/0003:046D:C05A.0001/input/input10
[ 1.323085] hid-generic 0003:046D:C05A.0001: input,hidraw0: USB HID v1.11 Mouse [Logitech USB Optical Mouse] on usb-0000:00:1d.0-1/input0
[ 1.384151] usb 1-5: New USB device found, idVendor=0bda, idProduct=0158
[ 1.384156] usb 1-5: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 1.384161] usb 1-5: Product: USB2.0-CRW
[ 1.384165] usb 1-5: Manufacturer: Generic
[ 1.384169] usb 1-5: SerialNumber: 20071114173400000
[ 1.491238] usb 1-6: new high-speed USB device number 3 using ehci-pci
[ 1.529239] usb 6-2: new full-speed USB device number 3 using uhci_hcd
[ 1.680288] usb 6-2: New USB device found, idVendor=19d2, idProduct=fff5
[ 1.680294] usb 6-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 1.680299] usb 6-2: Product: USB Storage
[ 1.680303] usb 6-2: Manufacturer: ZTE, Incorporated
[ 1.680307] usb 6-2: SerialNumber: 000000000002
[ 1.682910] usb 1-6: New USB device found, idVendor=0c45, idProduct=63ee
[ 1.682915] usb 1-6: New USB device strings: Mfr=2, Product=1, SerialNumber=0
[ 1.682919] usb 1-6: Product: Integrated_Webcam_1.3M
[ 1.682923] usb 1-6: Manufacturer: 
[ 2.334312] usb-storage 6-2:1.0: USB Mass Storage device detected
[ 2.346023] scsi host6: usb-storage 6-2:1.0
[ 2.346156] usbcore: registered new interface driver usb-storage
[ 2.349702] usbcore: registered new interface driver uas
[ 2.352412] ums-realtek 1-5:1.0: USB Mass Storage device detected
[ 2.370031] scsi host7: usb-storage 1-5:1.0
[ 2.370163] usbcore: registered new interface driver ums-realtek
[ 3.353337] scsi 6:0:0:0: CD-ROM ZTE USB Storage FFE9 2.31 PQ: 0 ANSI: 2
[ 3.356328] scsi 6:0:0:1: Direct-Access ZTE USB Storage FFE9 2.31 PQ: 0 ANSI: 2
[ 13.829750] input: Integrated_Webcam_1.3M as /devices/pci0000:00/0000:00:1a.7/usb1/1-6/1-6:1.0/input/input17
[ 13.829863] usbcore: registered new interface driver uvcvideo
[ 13.829865] USB Video Class driver (1.1.1)
[ 101.669363] usb 6-2: USB disconnect, device number 3
[ 111.160027] usb 6-2: new full-speed USB device number 4 using uhci_hcd
[ 111.307156] usb 6-2: New USB device found, idVendor=19d2, idProduct=fff5
[ 111.307162] usb 6-2: New USB device strings: Mfr=1, Product=2, SerialNumber=3
[ 111.307165] usb 6-2: Product: USB Storage
[ 111.307168] usb 6-2: Manufacturer: ZTE, Incorporated
[ 111.307170] usb 6-2: SerialNumber: 000000000002
[ 111.310244] usb-storage 6-2:1.0: USB Mass Storage device detected
[ 111.310439] scsi host8: usb-storage 6-2:1.0
[ 111.954256] usb 6-2: USB disconnect, device number 4
[ 113.410028] usb 6-2: new full-speed USB device number 5 using uhci_hcd
[ 113.555247] usb 6-2: New USB device found, idVendor=19d2, idProduct=ffe9
[ 113.555257] usb 6-2: New USB device strings: Mfr=1, Product=2, SerialNumber=0
[ 113.555262] usb 6-2: Product: ZTE CDMA Tech
[ 113.555266] usb 6-2: Manufacturer: ZTE, Incorporated
[ 113.575337] usb-storage 6-2:1.5: USB Mass Storage device detected
[ 113.575549] scsi host9: usb-storage 6-2:1.5
[ 113.675348] usbcore: registered new interface driver option
[ 113.675497] usbserial: USB Serial support registered for GSM modem (1-port)
[ 113.676298] usb 6-2: GSM modem (1-port) converter now attached to ttyUSB0
[ 113.676571] usb 6-2: GSM modem (1-port) converter now attached to ttyUSB1
[ 113.676820] usb 6-2: GSM modem (1-port) converter now attached to ttyUSB2
[ 113.677080] usb 6-2: GSM modem (1-port) converter now attached to ttyUSB3
[ 113.677325] usb 6-2: GSM modem (1-port) converter now attached to ttyUSB4
[ 114.581316] scsi 9:0:0:0: Direct-Access ZTE USB Storage FFE9 2.31 PQ: 0 ANSI: 2
Step-II (Check Whether Modem Manger Packages is installed or not)
root@techbrown ~ # rpm -q ModemManager
 ModemManager-1.4.6-1.fc22.x86_64
OR
root@techbrown ~ # dnf info ModemManager
Sample Output
Installed Packages
Name : ModemManager
Arch : x86_64
Epoch : 0
Version : 1.4.6
Release : 1.fc22
Size : 2.4 M
Repo : @System
Summary : Mobile broadband modem management service
URL : https://wiki.gnome.org/Projects/NetworkManager
License : GPLv2+
Description : The ModemManager service manages WWAN modems and provides a consistent API for
: interacting with these devices to client applications.
Step-III (Install the ModemManager Services if not installed)
root@techbrown ~ # dnf install ModemManager

How to fix this ?

Step-I (Start the ModemManager Service)
root@techbrown ~ # systemctl start ModemManager
Step-II (Add ModemManager to Systemd Service and System Manager)
root@techbrown ~ # systemctl enable ModemManager

Restart / Reboot the PC after that Plug in the USB Modem / Dongles .
Congratulations now you have enabled the Modem Manager Services.