canon ts6420a scanner lid is up but the error says 'check scanner unit' and it won't do anything

Diagnosis

The "Check Scanner Unit" error on the Canon PIXMA TS6420a typically indicates a failure in the communication loop between the CIS (Contact Image Sensor) assembly and the main logic board. While the error message suggests a physical obstruction or an open lid, the root cause is usually an "indexing error." This happens when the scanner head fails to return to its home position (the far left side) or cannot detect the home position sensor, causing the firmware to trigger a safety lockout to prevent the motor from grinding. This is often caused by a microscopic piece of debris or a smudge of ink/dust on the white plastic strip (the encoder strip) that runs parallel to the scanner glass. If the sensor cannot "read" its position on this strip, it assumes the unit is jammed or disconnected. In some cases, a power surge can cause the scanner carriage to "drift" slightly out of alignment, making the printer believe the lid is improperly seated or the unit is malfunctioning even when physically closed. You can confirm this by listening closely during startup; if you hear a rhythmic clicking or a grinding sound coming from the scanner bed before the error appears, the carriage is physically stuck.

How to Fix It

To resolve the indexing error and reset the scanner unit, follow these precise steps:

1. Power down the printer using the Power button on the control panel.
2. Unplug the AC power cord directly from the wall outlet and the back of the printer.
3. Open the scanner lid fully. Use a clean, dry microfiber cloth to gently wipe the glass, focusing on the far left edge where the scanner head rests.
4. Carefully inspect the thin, clear plastic strip (the encoder strip) located just behind the glass area. If you see any dust or ink splatters, use a dry cotton swab to gently clean it. Do not use rubbing alcohol or glass cleaner, as this can strip the timing marks off the strip.
5. Close the scanner lid and apply firm, downward pressure on all four corners. Ensure the lid is perfectly flush with the chassis; if the plastic hinges are slightly skewed, the sensor may not trigger.
6. Wait 60 seconds to allow the capacitors on the logic board to discharge completely.
7. Plug the power cord back into the wall first, then into the printer.
8. Turn the printer on and immediately press the "Stop" button if the error reappears, then press "OK" to attempt a manual reset of the carriage.

If That Didn't Work

• Perform a Hard Hardware Reset: While the printer is on, unplug the power cord from the back. Hold the Power button for 30 seconds to drain residual electricity. Plug it back in and immediately hold the "Stop" button for 5 seconds. This forces the scanner carriage to perform a full-range calibration sweep.
• Check for "Ghost" Jams: Open the printer's ink cartridge access cover. Even though the error says "scanner unit," a jammed piece of paper in the main tray or a displaced carriage can occasionally trigger a generic "unit error" across the bus. Clear any scraps of paper from the rear feed or the output tray.
• Update Firmware via USB: If the error is software-based, the wireless connection may be blocked by the error state. Connect the TS6420a to a PC via a USB-B cable. Use the Canon update utility to force a firmware flash. This can overwrite corrupted sensor calibration data.
• Inspect the Hinge Cables: Check the rear of the printer where the scanner lid hinges are located. Ensure no cables are pinched or protruding. If the internal ribbon cable has become loose due to vibration, the printer will report a "check unit" error because it cannot see the scanner head.

Heads Up

• Avoid Force: Never attempt to manually slide the scanner head (the light bar) while the printer is powered on, as this can permanently damage the stepper motor or strip the drive gears.
• Dust Prevention: Keep the scanner lid closed when not in use. Dust accumulation on the encoder strip is the number one cause of "Check Scanner Unit" errors on the TS series.
• Power Stability: This model is sensitive to voltage drops. If you are using a low-quality power strip or extension cord, plug the printer directly into a wall outlet to ensure the scanner motor receives full current for its home-position sweep.
• Cleaning Agents: Never spray liquid cleaners directly onto the scanner glass. Liquid can seep into the edges of the bezel and corrode the CIS sensor, leading to permanent vertical lines in your scans or a total unit failure.