F03 error code, water temperature overshoots setpoint or unit shuts down on overheat

Safety first: Turn OFF power at the breaker. Allow the tank to cool completely (water may be scalding).

Steps to check:

• Remove upper and lower access panels (electric) or front panel (gas hybrid models) to access thermostats and elements.
• Press the red reset button on the manual reset high-limit thermostat (usually on the upper thermostat or control board assembly). A distinct click indicates it was tripped.
• Inspect wiring to elements and thermostats for signs of overheating or loose connections.
• Check for excessive scale buildup by removing one element and inspecting for heavy mineral deposits.

Further diagnostics:

• Use a multimeter to verify that the thermostat opens and closes correctly at the specified temperatures (refer to service manual).
• If the high limit trips repeatedly, suspect a stuck relay on the control board or a thermostat that is not regulating properly.

Corrective actions:

• Descale or replace heavily scaled heating elements.
• Replace faulty thermostat or control board if it is not cycling power correctly.
• After repairs, reset the high limit and monitor operation.

Warning: Do not bypass or defeat high-limit safety devices. This is a serious scald and fire hazard.