Robot lawnmower shows E04 and refuses to start; often occurs after rain or washing.

Safety first: Turn OFF the main power switch. Do not operate the mower on wet grass or in rain.

• Dry the mower: Move the mower to a dry, sheltered area. Allow it to dry for several hours. Wipe off visible moisture from the top cover and underside.
• Clean rain sensor: Locate the rain sensor area on the top housing (often a small recessed grid or metal pads). Gently clean it with a soft cloth and a small amount of isopropyl alcohol to remove dirt and residues.
• Inspect under the cover: Remove the top cover and check for water droplets or condensation on the mainboard and sensor connectors. Dry carefully with a lint‑free cloth; do not use heat guns.
• Check connector: Ensure the rain sensor connector is firmly seated on the mainboard and shows no corrosion.
• Reset: After drying, turn power ON and clear any error via the control panel or app, then attempt a test run.

If E04 persists in dry conditions, the rain sensor module or its input circuit on the mainboard may be defective and require replacement.