Heater shows E05 and does not heat; sometimes accompanied by clicking relays but no power to heating elements.

Safety: Turn off all breakers feeding the heater. Verify all phases are de-energized with a meter before touching internal components.

• Visual inspection: Remove the cover and inspect heating element terminals for discoloration, melted insulation, or loose screws. Tighten any loose connections to manufacturer torque specs.
• Check supply voltage: With power on (only if you are qualified), verify correct line voltage at the input terminals and that all phases are present on 3-phase units. Turn power off again before proceeding.
• Test elements: Disconnect each heating element and measure resistance with a multimeter. Compare to the expected value from the service manual. Replace any element that is open or significantly out of spec.
• Inspect relays/triacs: Look for burn marks or obvious damage on the power board. If elements test good but E05 persists, the switching devices on the board may be faulty.
• Restore and test: After replacing any failed element, reassemble, restore power and water, and test. If E05 remains, the main control/power board likely needs replacement by a qualified technician.