When a water heater is powered on but fails to heat water, the issue is typically caused by component failure or safety cutoffs. On electric water heaters, burned-out heating elements, a tripped high-limit thermal reset switch, or a faulty thermostat will prevent heat generation even while power flows to the unit. On gas water heaters, an extinguished pilot light, a broken thermocouple blocking the gas valve, or a malfunctioning gas control valve stops the burner from igniting. Verification: Test electrical resistance across heating elements with a multimeter or check gas burner flame status.
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