Oso water heater shows E02 and shuts down shortly after calling for heat.

Safety first: Switch OFF power at the breaker and verify no voltage at the heater before opening any covers.

• Locate the safety thermostat: Remove the front service panel. On many Oso models, the manual reset safety thermostat is mounted on or near the element pocket or on the control plate.
• Reset the thermostat: Press the small red or black reset button on the safety thermostat until it clicks. If it immediately trips again when powered, do not keep resetting—find the cause.
• Check the control thermostat: With power off, inspect the adjustable thermostat sensor (often inserted into a dry pocket). Ensure it is fully inserted and not bent. If the thermostat contacts are welded closed (heater runs continuously until overheat), replace the thermostat.
• Inspect for limescale: Hard water can cause heavy scale on the element, leading to localized overheating. Drain the cylinder to below the element, remove the element, and inspect. Descale or replace the element as needed.
• Verify circulation: On indirect or combi Oso units, ensure any circulation pump and check valves are operating correctly so hot water is not trapped around the sensor.
• Reassemble and test: Refit components, restore water and power, and monitor temperature. Set the thermostat to the recommended setting (typically 55–60°C) to avoid nuisance trips.

If E02 recurs frequently, have a technician check the control board and sensor calibration.