DIAGNOSIS: Error 3 is a false positive from the pad tray sensor. Debris or moisture is likely interfering with the magnetic sensor in the tray slot.
FIX: Remove the pad tray. Inspect the small rectangular slot inside the robot where the tray inserts. Use a dry cotton swab to clean any dirt or dried cleaning solution from the slot and the two small metal contacts. Let it air dry for 5 minutes. Reinsert the tray firmly until it clicks.
IF THAT DIDN'T WORK: The pad tray itself might be faulty. The magnet inside the plastic tray can dislodge. Shake the tray near your ear; you shouldn't hear anything loose. If you do, you need a replacement pad tray (part # AFA100).
HEADS UP: Never run the m6 without a pad tray installed, even for a test. It can damage the water pump.