my dreame l20 ultra keeps saying 'please clean the mop pads' over and over even after i just washed them.

Diagnosis

The Dreame L20 Ultra utilizes a sophisticated optical dirt-detection system integrated into the base station's washing platform. This system uses infrared sensors to analyze the light reflectivity and turbidity of the water as it washes the mop pads. When the robot reports "please clean the mop pads" repeatedly despite the pads being visually clean, it is typically due to "sensor ghosting." This happens when a thin film of dried detergent, mineral scale from hard water, or a buildup of fine sediment remains on the sensor windows located at the bottom of the base station's washing tray. Because the sensors are calibrated for high precision, even a microscopic layer of soap scum can refract the light, tricking the system into believing the water is still murky or the pads are saturated with grime. Another common cause is the improper seating of the mop pads; if the Velcro is worn or the pad is slightly offset, the robot cannot achieve the correct pressure against the scrubbing mechanism, leading to an incomplete cleaning cycle that triggers the alert. You can confirm this by checking the "Dirty Water Tank"—if the water looks clear but the alert persists, the issue is sensor interference, not actual dirt.

How to Fix It

To resolve this, you must perform a deep clean of the sensor array and reset the robot's detection logic. Follow these exact steps:

1. Power down the L20 Ultra completely using the physical power button on the top of the chassis. Ensure the unit is fully off and not in standby mode.
2. Remove both mop pads from the robot. Inspect the Velcro strips on the underside of the robot; if there is lint or hair trapped in the hooks, use a fine-tooth comb to clear them so the pads sit flush.
3. Locate the washing tray in the base station. Remove the removable tray (if applicable to your region's model) and look for the small, clear plastic windows located near the water drainage area. These are the optical sensors.
4. Using a microfiber cloth dampened with 70% isopropyl alcohol, gently wipe these sensor windows. Do not use abrasive scrubbers, as scratches on the plastic will cause permanent false readings.
5. Wash the mop pads by hand using only warm water. Avoid using fabric softeners or heavy detergents, as these leave a chemical residue that alters the water's reflectivity during the automated wash cycle.
6. Dry the pads completely or ensure they are firmly pressed into the robot's mounting brackets without any bunching.
7. Power the robot back on and initiate a "Mop Wash" cycle from the app to recalibrate the sensor baseline.

If That Didn't Work

• Deep Clean the Base Station Drain: Debris can clog the narrow drainage pipe leading to the dirty water tank. If water pools in the tray for too long, the sensors detect "stagnant" water and trigger the cleaning alert. Use a flexible pipe cleaner or a small vacuum to clear the drainage port at the bottom of the washing tray.
• Check the Water Tank Float: Ensure the float sensor in the dirty water tank is moving freely. If the float is stuck in the "full" position due to slime buildup, the robot may erroneously report that the mop pads cannot be cleaned because the waste water cannot be evacuated.
• Update Firmware via App: Check for a pending firmware update in the Dreamehome app. Several updates for the L20 Ultra specifically addressed the sensitivity thresholds of the mop-dirt detection algorithm to reduce false positives in homes with hard water.
• Hard Reset the Base Station: Unplug the base station from the wall outlet for 60 seconds. This clears the temporary cache of the base station's onboard controller, forcing it to re-poll the sensor data from scratch when the robot returns for its next wash.

Heads Up

• Avoid Third-Party Detergents: Only use the official Dreame cleaning solution. Generic floor cleaners often create excessive foam; this foam creates air bubbles that block the optical sensors, triggering the "clean mop pads" error.
• Hard Water Maintenance: If you live in a hard water area, descale the base station tray once a month with a mild citric acid solution to prevent calcium buildup on the sensors.
• Pad Replacement Cycle: Replace your mop pads every 3-6 months. As the fabric degrades, it holds onto more grime and loses its shape, which prevents the sensors from accurately reading the "cleanliness" of the pad.
• Tank Hygiene: Always empty and rinse the dirty water tank immediately after it is full. Leaving dirty water to sit creates a biofilm that can seep back into the sensor housing.