Oso water heater shows E04 and stops heating; unit may have recently overheated or been drained.

Safety first: Turn OFF electrical power and close the water supply before working on the heater.

• Verify cylinder is full: Open a nearby hot water tap fully. You should get a strong, steady stream of water without sputtering. If air is present, allow it to run until all air is purged.
• Check inlet and outlet valves: Ensure the cold inlet isolation valve and any check valves on the Oso cylinder are fully open and not blocked.
• Reset protection (if provided): Some Oso models have a manual reset for dry‑fire or overheat on the control plate. Press the reset button after confirming the tank is full.
• Inspect wiring: Remove the front cover and verify that any level or flow interlock wiring is correctly connected and not bypassed or damaged.
• Check element condition: If the element was dry‑fired, it may be damaged. With power off and tank drained below the element, remove and visually inspect the element for bulging, cracks, or burn marks. Replace if suspect.
• Refill and bleed: Refill the cylinder completely, bleed air from all hot taps, then restore power and confirm E04 clears.

If the code returns even with a full cylinder and good element, the control board’s dry‑fire detection circuit may require replacement by a technician.