How to Fix Lexmark "900 Firmware Error"

The test that tells you which kind of problem you have

Turn the printer off, disconnect every cable except the power cord (network cable, USB, everything), and turn it back on. Whether the error still appears with nothing but power connected is the key diagnostic:

• If the error disappears with cables disconnected: it's a network or communication issue, not a hardware fault. Reconnect the cables one at a time and check the network side (router, switch, cable itself) before assuming anything about the printer.
• If the error persists with nothing connected: this points to a genuine hardware or firmware-level fault inside the printer itself.

If it's the communication/network type

Check whether the error only happens when a specific job or a specific computer sends a print request — if so, this points toward a driver or job-compatibility issue on that computer rather than the printer itself. Also check whether the printer sits on a power strip, surge protector, or older UPS — some of these can cause power fluctuations that trigger a false 900 error, particularly if the error appears intermittently rather than consistently.

If it persists with everything disconnected

Try a firmware update first — download the current firmware for your exact model from Lexmark's official support site and apply it (ensure the printer has stable, uninterrupted power throughout the update process, since an interrupted firmware update can make things worse rather than better). If the error clears after updating, you're done. If it returns immediately even after a successful update with nothing connected, this points to a mainboard-level hardware fault that genuinely requires professional service — this isn't something further troubleshooting at home will resolve.

One more thing worth checking first

Before assuming hardware, print a menu settings page (usually accessible via the printer's own control panel under Reports or Menu Settings) if the printer is responsive enough to do so — this reveals device and firmware version information under "Base" or "Engine" that can help confirm whether you're running current firmware, which matters before you decide whether an update is the missing step.