Printer troubleshooting (personal notes)
Printers are notoriously unreliable. Here, I record printing issues I've encountered and how I fixed them. This saves me time if I encounter them again a year or two later and don't recall the solution. Perhaps it will also be useful for other people who encounter the same issues.
I'm currently using a Brother HL-L2300D with CUPS.
Ensure printer is on and auto power off is disabled
There have been times when my printer was off and I didn't notice. This happened because I had accidentally enabled auto power off.
Print a copy of the "printer settings" page: turn off the printer, turn it back on, then press the Go button three times. Check if AUTO POWER OFF is set to "OFF".
If it isn't, press Go four times. All the LEDs will blink twice when auto power off is enabled, or once when it is disabled. To confirm, print another copy of the "printer settings" page.
Ensure printer is not paused
Open the CUPS administration page at http://localhost:631/printers/. If the status is "Paused", click the printer's name, then do Maintenance → Resume Printer.
Duplex/double-sided printing doesn't work from Gnome print settings
The Gnome print settings seem to be ignored. Instead, find the printer on http://localhost:631/printers/, then go to Administration → Set Default Options. Set Duplex to "DuplexNoTumble".