Brother lasers warn when toner is low, then stop at "Replace Toner". If there's still toner, you can often continue printing with the toner-low override.
Work through these in order; most people are fixed by the first two or three.
Remove the toner, gently rock it side to side to redistribute, and reinstall — this can extend a low cartridge.
In the printer's settings, enable "Continue" so it keeps printing past the toner-low warning until quality drops.
After fitting new toner, follow your model's reset step (open/close the front cover, or the panel reset sequence) so the sensor recognizes it.
Some cartridges have a clear plastic window the sensor reads — make sure it isn't covered or dirty.
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