Serial port unavailable
What “Serial port unavailable” usually means
When PuTTY reports Serial port unavailable, the client has already attempted a network or protocol step and failed before a normal interactive shell is available. This guide explains what the message usually means on Windows clients, which local settings to verify first, and when the problem is almost certainly on the server, firewall, or DNS path rather than inside the PuTTY GUI.
PuTTY serial mode cannot open the selected COM port—device missing, in use, lack of permissions, or wrong port name.
Independent resource: Putty.info is educational only and is not affiliated with the official PuTTY project. Compare symptoms against the official documentation for your installed version.
Symptoms you will see
Operators usually notice this failure in one of a few repeatable ways. Capture the exact dialog text, the hostname and port from the Session panel, and whether the failure happens before or after a username prompt—those details decide which checklist below applies.
- Error opening COM port for serial sessions.
- Works in Device Manager appearance but still locked by another app.
Likely causes ranked by frequency
Several independent conditions can produce the same client-facing wording. Work from the outside in: reachability first, protocol selection second, authentication third. Changing key files will not fix a TCP refusal, and opening a wider firewall will not fix a rejected public key.
- USB-serial adapter unplugged or driver missing.
- Another terminal holds the port exclusively.
- Wrong COM number after replug.
- Insufficient Windows permissions on locked-down images.
Step-by-step client checks
Use this ordered checklist on a workstation you control. Prefer saved sessions so Host Name, Port, and Connection type stay consistent while you isolate one variable at a time.
- Confirm Connection type is Serial and the Serial line matches Device Manager.
- Close other serial tools (including other PuTTY windows).
- Re-seat USB adapters; note COM number changes.
- Match speed/parity/flow control to the device documentation.
- Update or reinstall vendor USB-serial drivers from the hardware maker—not random bundles.
- See /learn/serial-console/ for session setup.
Do not paste private keys, passphrases, or production passwords into Putty.info tools or contact forms. Diagnose locally with PuTTY, Plink, PSCP, or PSFTP from an official install.
When it is a server or network issue
For network serial concentrators, verify the appliance mapping separately; this error is usually local to the Windows COM namespace.
If TCP connects from another network path but fails from yours, involve the network team with traceroute/path evidence rather than repeatedly regenerating keys. If authentication fails after a banner appears, collect the server sshd logs for the same timestamp and username—client-side retries alone rarely reveal AccountLocked, Match blocks, or AllowUsers denials.