Tesy water heater displays E04 and stops; may occur after power surge or lightning.

Safety first: Turn off the breaker and verify no voltage at the heater. Do not work on the control board while energized.

Steps:

• Power cycle: Disconnect power for 5–10 minutes to allow capacitors to discharge. Restore power and check if E04 clears. If it returns, continue.
• Inspect control board: Remove the front cover and locate the electronic control module. Check for burn marks, swollen capacitors, or a burnt smell.
• Check connectors: Reseat all low-voltage connectors (sensor, display, keypad) and high-voltage connectors (element, mains input). Ensure firm contact.
• Surge protection: If E04 appeared after a storm or grid event, consider installing an external surge protector for the heater circuit after repair.
• Replace control board: If visible damage is present or E04 persists after reseating connectors, replace the Tesy electronic control board with the exact model-specific part. Transfer any jumpers or configuration plugs as per service manual.
• Reassemble and test: Refit covers, restore power, and verify normal operation and temperature control.

Warning: Board-level component repair (soldering, chip replacement) should only be done by qualified electronics technicians; otherwise, replace the entire module.