Why does my OctoPrint printer show as "Not connected"?

TL;DR

Your printer can't talk to OctoPrint over USB. SimplyPrint just reports what OctoPrint reports - we can see that the connection isn't happening, but the fix is not on our side. Almost always it's the USB cable, USB power, the printer firmware, or a printer-specific quirk OctoPrint needs help with. The fastest path to a fix:

Open OctoPrint directly at http://<pi-ip-address> and read the Connection panel + Terminal tab. That's where the actual error lives.
Try a different data-grade USB cable.
If you're running an Anycubic, Creality, Anet, or a printer with a finicky USB board, see printer-specific quirks below.
If you've worked through this article and it still won't connect, the best places for help are the OctoPrint Community forum (community.octoprint.org) and the OctoPrint Discord - they have far more eyes on this layer of the stack than SimplyPrint support does.

This is not a SimplyPrint bug, and SimplyPrint support can't fix it. "Not connected" means OctoPrint itself cannot reach your printer over the USB cable. SimplyPrint mirrors OctoPrint's connection state: if OctoPrint can't connect, neither can SimplyPrint. The fix is between OctoPrint, the Pi, the USB cable, and the printer - all layers we don't control. We can point you at the right diagnostics, but the actual fix happens on your side.

1. Step 1: Open OctoPrint directly

Before anything else, open OctoPrint in your browser at http://<pi-ip-address> (or whatever address you set up). OctoPrint will tell you exactly what's failing, because it's the layer talking to the serial port.

Read these in OctoPrint:

Where	What it tells you
Connection panel (top left)	The serial port, baud rate, and the exact error. Common ones: "No serial port found", "Connection error", "Failed to autodetect baudrate".
Terminal tab	What the printer is (or isn't) sending. An empty terminal means nothing's coming through the cable. Garbled text means baud rate is wrong.
State	If it says "Offline" or "Closed" here too, the issue is confirmed to be below OctoPrint - not SimplyPrint.

If OctoPrint can connect to the printer but SimplyPrint still shows "Not connected", that's a different problem - see Why is my OctoPrint-powered printer offline? instead.

2. Quick checklist

Is the printer powered on?
Try a different USB cable - it must be a data cable, not charge-only (more on this below).
Try a different USB port on the Pi.
Power-cycle the printer (off, wait 30 seconds, on).
Restart the Pi.

If none of that fixed it, read on.

3. Bad USB cable (about 9 out of 10 cases)

Many USB cables are charge-only and can't transmit data. They look identical to data cables from the outside.

Signs of a bad cable:

Very thin, no shielding
Bundled with a phone charger or budget gadget
No markings or branding
You've had flaky behaviour with it before

Fix: A known-good shielded USB cable, preferably with ferrite cores (the cylindrical bumps near the ends) which reduce interference from the printer's stepper motors.

Reference: Which cable to use for SimplyPrint / OctoPrint

4. USB power: backpowering and undervoltage

This is a real, well-documented issue with some printers. Worth knowing about because it produces the exact same "Not connected" symptom and random Pi reboots / undervoltage warnings.

What backpowering is

On certain printer mainboards (older Creality Ender 3 boards, Anet A8, Anycubic Mega, and others), the 5V rail on the USB port is tied to the mainboard's 5V supply. When connected to a Pi:

The Pi can feed power into the printer mainboard - keeping the printer's LCD lit even when the printer is "off"
The printer mainboard can feed power back into the Pi - causing undervoltage detection, random reboots, and dropped USB connections mid-print

Either direction causes the same end result for you: "Not connected" in OctoPrint, intermittently.

Symptoms

Printer LCD stays on when the printer power supply is switched off
Lightning-bolt undervoltage icon in OctoPrint's top bar
Pi randomly reboots, especially when the printer's heated bed or hotend turns on
Connection drops mid-print

Fixes (best to worst)

USB isolator (best). A hardware device sitting between Pi and printer that electrically isolates the two sides. About $20.
Powered USB hub between Pi and printer. Cheaper, works well, and also gives you more ports.
Tape over the USB +5V pin on the cable (pin 1 on the Pi side). A common community fix - blocks power transfer without affecting data. Some sellers also list "data-only" cables that have this done for you.
Power-on sequence: turn the printer on first, then connect the USB cable. Helps in mild cases.
Better Pi power supply (5.25V / 3A+ official Raspberry Pi supply) if undervoltage is your symptom. Doesn't fix backpowering, but increases margin.

Don't use a phone charger to power the Pi. They almost always trigger undervoltage under load.

Background reading: OctoPrint Community: printers known to have backpowering issues.

5. Printer-specific quirks and OctoPrint plugins

Some printers don't speak the standard Marlin protocol cleanly, and OctoPrint can struggle to autodetect or stay connected without a printer-specific plugin or manual baud rate. This is something the OctoPrint Community has documented extensively per printer model - if you've tried the steps above and you have one of these printers, search the OctoPrint forum for your printer model.

Printer family	Common quirk	Typical fix
Anycubic i3 Mega / Mega S	Non-standard `M115` / `OK` responses; baud rate autodetect fails	Manually set baud to 250000 in OctoPrint; install a community Anycubic-specific plugin if available
Creality Ender 3 / CR-10 (older boards)	Default baud is 115200, newer "silent" boards use 250000	Manually set baud; older boards sometimes need a specific cable to pass through the USB filter on the board
Anet A8 / clones	Custom serial config, sometimes needs Marlin reflash	Community forum has model-specific guides
Prusa MK3 / MK3S / MK3S+ / MK3.5 / MK3.9	"RPi port" setting on the printer disables the regular USB port	On the printer: Settings > RPi port > off.

If Autodetect baudrate is failing in OctoPrint, don't keep retrying it. Set the baud manually. Try 250000 first, then 115200, then 230400.

6. Bad printer mainboard (rare)

Rare but real, especially after a failed firmware flash or a USB port that's been physically stressed. If you've ruled out everything above and the printer's USB port still doesn't respond, the mainboard may need replacing.

Quick test: plug the same cable into a different printer using the same Pi. If that one connects fine, your original printer's mainboard is the suspect.

7. Wrong setup guide (no USB port on the printer?)

Cable can't fit your printer? You're probably using the wrong setup guide.

OctoPrint is for Marlin-based printers that connect to a Raspberry Pi over USB. If your printer:

Runs Klipper natively (most modern Creality K1/K2, Elegoo Centauri, etc.)
Doesn't have a USB-B / Mini USB / USB-C port for printer connection
Is a Bambu Lab, Prusa with built-in WiFi (MK4 / XL / Core One), or an Ultimaker

...then OctoPrint is the wrong setup path. Pick the correct one at simplyprint.io/setup-guide.

Quick firmware test: look at the menu on your printer screen. If the menu looks like a typical Marlin LCD (text-only, brand-neutral options), OctoPrint is the right path. If it's a custom UI (Klipper screens like KlipperScreen, or brand-specific UIs like Bambu's), use the recommended setup for that brand instead.

Still stuck? Where to get help

We mean this honestly: the OctoPrint community is the right place for this, not SimplyPrint support.

OctoPrint Community forum - community.octoprint.org. Searchable, deep, model-specific archives. The answer to your problem is probably already there.
OctoPrint Discord - linked from the OctoPrint Community forum. Faster for back-and-forth diagnosis.
SimplyPrint Discord - simplyprint.io/discord. Good if you want a second pair of eyes from us, but understand that for cable / power / firmware / plugin issues we're going to point you back to OctoPrint anyway.

If you do post for help anywhere, include a screenshot of the OctoPrint Connection panel + Terminal tab. That single screenshot will get you a faster, better answer than any description of the symptom.

Updated on: 25/05/2026

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