it says 'close top cover' but the top cover is definitely fully closed and latched.

Diagnosis

The "Close Top Cover" error on the Brother MFC-L2820DW is triggered by a failure in the interlock circuit. This printer uses a mechanical micro-switch located on the chassis that is depressed by a plastic actuator tab attached to the underside of the top cover. When the cover is closed, this tab pushes the switch, completing the circuit and signaling the logic board that it is safe to operate. If the printer continues to report the cover is open despite being latched, the circuit remains open.

This failure typically stems from one of three root causes: the plastic actuator tab on the cover has snapped or worn down, the internal spring mechanism of the micro-switch (Brother part # SW4109001) has collapsed, or dust and toner debris have infiltrated the switch housing, preventing the contact from fully closing. You can confirm this by looking at the underside of the top cover for any missing plastic shards or by manually pressing the switch with a non-conductive tool to see if the error message clears on the LCD screen.

How to Fix It

To resolve this, you must access the internal interlock switch and either clean, adjust, or replace it. Follow these precise steps:

1. Power down the printer and unplug the power cable from the wall outlet to prevent electrical shock.
2. Open the top cover and remove the toner cartridge and drum unit assembly. Set them aside on a clean sheet of paper to avoid toner spills.
3. Locate the two screws on the right-hand side of the chassis that secure the outer plastic casing. Use a Phillips #2 screwdriver to remove them.
4. Carefully slide the right side cover panel backward and lift it away from the frame. This exposes the internal frame where the interlock switch is mounted.
5. Inspect the plastic actuator tab on the underside of the top cover. If it is broken, you will need to replace the top cover assembly. If it is intact, proceed to the switch.
6. Locate the micro-switch (a small black rectangular component with a metal lever). Use a can of compressed air to blow out any toner dust from around the lever and the contact points.
7. If the switch is faulty, remove the two small mounting screws holding the switch in place. Disconnect the wiring harness by gently pulling the connector straight back.
8. Install the replacement Brother part # SW4109001. Ensure the metal lever is facing the correct direction to meet the actuator tab.
9. Reconnect the wiring harness, secure the switch with screws, and slide the right side cover back into place.
10. Reinstall the drum and toner, close the cover firmly, and power on the device.

If That Didn't Work

• Check for Cover Warp: If the printer has been subjected to high heat or a heavy impact, the top plastic casing may be slightly warped. This prevents the actuator tab from reaching the switch even when latched. Try applying firm downward pressure on the right side of the cover while booting up; if the error disappears, you may need to add a thin plastic shim (such as a piece of a credit card) to the actuator tab to bridge the gap.
• Inspect the Wiring Harness: Trace the wires from the interlock switch back to the main logic board. Check for any pinched wires or loose connections that may have occurred during shipping or previous maintenance. A loose pin in the connector will mimic an "open cover" state.
• Perform a Hard Reset: Occasionally, the logic board retains a "ghost" error. Unplug the power cord and hold the power button for 30 seconds to discharge the capacitors. Plug it back in and restart.
• Verify Drum Seating: Ensure the drum unit is pushed completely into its slot. If the drum is slightly skewed, it can put upward pressure on the top cover, preventing the latch from engaging the switch fully.

Heads Up

• Toner Caution: Always handle the drum unit with care; avoid touching the green roller with your fingers, as skin oils can cause streaks on your printed documents.
• Avoid Lubricants: Never apply WD-40 or oil to the interlock switch. These attract toner dust, which will create a conductive sludge that can short-circuit the switch or cause it to stick permanently.
• Screwdriver Choice: Use a magnetized Phillips head screwdriver. The screws inside the MFC-L2820DW are small and easily dropped into the internal gears, which can cause permanent mechanical failure.