Brother HL 5370DW - Manual (Page 134)

Troubleshooting
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g Put the toner cartridge back into the drum unit until you hear it lock into place. If you put the cartridge in
properly, the blue lock lever will lift automatically.
h Open the front cover. Put the drum unit and toner cartridge assembly back in the printer.
i
Put the paper tray firmly back in the printer.
j Close the front cover.
k Make sure that the Paper LED is off and then, the Status LED lights green.