Control board powers up but displays E0 and will not start heating cycle

Safety first: Turn off the water heater’s power at the breaker (electric) or set gas control to OFF and unplug any power cord (gas models with electronic control). Allow the unit to cool before removing any covers.

• Check power supply: Verify the dedicated breaker is ON and delivering correct voltage (typically 120 V for control, 240 V for electric elements). If you are not trained, use a qualified electrician.
• Inspect control wiring: Remove the front access panel to expose the Giant control board. Check the low‑voltage harness connectors (thermostats, sensors, gas valve, igniter) for loose or partially seated plugs. Reseat each connector firmly.
• Reset the control: With power OFF for at least 60 seconds, restore power and see if E0 clears. Some Giant models have a small reset or test button on the control board—press and hold for 5–10 seconds if present.
• Check for moisture or corrosion: Look for signs of water intrusion or corrosion on the control board. If present, the board is likely damaged and must be replaced.
• Replace control board if needed: If wiring and power are correct and E0 persists, the Giant electronic control board is likely defective and should be replaced by a qualified technician.