Windows is notorious for holding onto COM port handles. If another application—or a crashed instance of QXDM—is holding the port open, QXDM will fail to create the service.
If the port shows up in Device Manager but QXDM still gives the "Failed to create diag service" error, the driver stack is likely corrupt. Simply updating the driver often isn't enough; you must uninstall it cleanly.
To fix the error, one must first understand the architecture. When you launch QXDM and attempt to communicate with a device, you are initiating a request to the device's subsystem. This is a low-level software layer running on the Qualcomm chipset. qxdm failed to create diag service
The DIAG port can only be opened by one software at a time. If QPST Software Download, QCAT, or even a serial terminal like Putty is holding the port, QXDM will fail.
| Symptom | Check Command / Action | Expected Result | |---------|------------------------|------------------| | No DIAG port in Device Manager (Windows) | Check USB descriptor: lsusb -v \| grep -A 10 "Diagnostic" | Interface class 0xFF, subclass 0xFF, protocol 0x23 | | Port present but no response | Send raw DIAG_CMD_VERSION_QUERY (0x00) via serial | Reply with version, max packet size | | DIAG_CMD_SUBSYS returns error | Trace using internal UART log (if accessible) | Look for diag_comm_subsys_handler: svc not registered | | Modem reset on DIAG command | Check QDSS (Qualcomm Diagnostic Subsystem) logs via JTAG | DIAG task exception or null pointer dereference | Windows is notorious for holding onto COM port handles
Navigate to the tab and click Refresh to ensure your QXDM Professional license is active and valid. 4. Administrative Privileges
To avoid this error in future projects:
Sometimes the PC is ready, but the device is not exposing the port. Android devices