getting 'paper jam open rear door' but there's no paper stuck anywhere i can see

Diagnosis

The "Paper Jam Open Rear Door" error on the HP LaserJet Pro M404dn is frequently a "ghost jam." This occurs when the printer's optical sensors—specifically the rear exit sensor and the duplexing path sensor—are triggered or blocked, even if no full sheet of paper is visible. Because this model uses a high-tension paper path, a tiny fragment of paper (sometimes as small as 2mm) or a buildup of accumulated paper dust can obstruct the sensor's beam, leading the logic board to believe a jam has occurred. Another common cause is a fatigued or misaligned sensor flag. The sensor consists of a small plastic "finger" (the flag) that moves back and forth as paper passes. If this flag becomes dislodged from its pivot or gets stuck in the "closed" position due to a piece of adhesive from a label or a torn scrap of 20lb bond paper, the printer will trigger the rear door error immediately upon startup or mid-print. You can confirm this by opening the rear door and observing if the plastic flags snap back instantly when flicked gently with a finger; if they feel sluggish, there is debris in the hinge.

How to Fix It

To resolve this, you must perform a deep clean of the duplexing assembly and the rear exit path. Follow these precise steps:

1. Power down the printer and unplug the power cable from the wall to prevent electrical shorts when probing the sensor area.
2. Open the rear access door fully. Locate the green duplexer release levers on both the left and right sides of the rear chassis.
3. Squeeze both green levers simultaneously and pull the entire duplexer assembly (the large plastic tray/guide) straight out of the machine. Set it aside on a clean surface.
4. Using a high-intensity LED flashlight, inspect the cavity where the duplexer was seated. Look specifically at the rear-most rollers. Use a pair of needle-nose tweezers to remove any tiny white shreds or "confetti" paper stuck in the crevices.
5. Locate the plastic sensor flags—small, black plastic levers—near the exit rollers. Gently flick them with your finger to ensure they spring back into place. If they are stuck, use a can of compressed air to blow out any dust from the pivot point.
6. Inspect the rollers inside the duplexer assembly you removed. Wipe the rubber rollers with a lint-free cloth dampened with distilled water to remove paper dust that may be causing slippage, which can trigger false jam readings.
7. Slide the duplexer assembly back into the chassis until you hear a definitive "click" on both sides. Close the rear door firmly.
8. Plug the printer back in and perform a "Warm Up" cycle to reset the sensor state.

If That Didn't Work

• Perform a Hard Power Reset: While the printer is ON, pull the power cord directly from the back of the unit. Wait exactly 60 seconds. This drains the capacitors on the formatter board and clears the volatile memory that may be "holding" the error state. Plug it back in and restart.
• Clean the Pickup Rollers: If the jam is actually occurring just before the rear door, the pickup rollers may be slipping. Use a 90% Isopropyl Alcohol swab to clean the rubber rollers in the main tray. If the rubber is shiny and smooth (glazed), use a very fine abrasive pad to lightly scuff the surface to restore grip.
• Check the Paper Weight Settings: If you are using heavy cardstock or labels, the printer may be timing out before the paper reaches the rear sensor. Go to the control panel: Settings > Paper Setup > Paper Type and ensure it is set to "Heavy" or "Cardstock" rather than "Plain Paper." This slows the motor speed and prevents false timing jams.
• Inspect the Rear Door Hinges: Check the plastic tabs on the rear door itself. If a tab is snapped or bent, the door may not be seating flush against the chassis, leaving the door-closed sensor open by a fraction of a millimeter, which triggers the "Open Rear Door" message.

Heads Up

• Never use a screwdriver or metal knife to scrape out paper jams; you risk scratching the drum or piercing the fuser film, which will cause permanent print streaks and require a costly fuser replacement.
• Avoid using oil-based lubricants or WD-40 on the sensor flags or rollers, as this will attract paper dust and cause the rollers to slip, leading to actual jams.
• Always use high-quality, moisture-free paper. Damp paper curls as it passes through the fuser, increasing the likelihood of "ghost jams" in the rear duplexer.
• If the error persists after a deep clean, the rear sensor may have failed electrically. This requires a technician to replace the sensor assembly on the main logic board.