Digital Oso water heater will not heat; display shows E01 and water remains cold.

Safety first: Turn OFF power to the Oso water heater at the breaker and verify with a multimeter that there is no voltage at the heater terminals before removing any covers.

• Access the element: Remove the front service cover on the Oso cylinder, then remove the small access cover over the lower heating element.
• Inspect wiring: Check for burned, loose, or discolored wires on the lower heating element terminals. Tighten loose screws and replace any heat-damaged wire with the correct high‑temperature cable.
• Test the element: Disconnect both wires from the element. Set a multimeter to ohms and measure across the two element terminals. A typical 2–3 kW Oso element should read between roughly 18–30 Ω (check your rating plate). If the meter shows OL (open) or 0 Ω (short), the element is faulty.
• Check for earth leakage: Measure between each element terminal and the metal flange/body. Any continuity indicates a ground fault and the element must be replaced.
• Replace the element: Isolate water supply, close inlet valve, and relieve pressure via a hot tap. Drain the cylinder to below the element level. Remove the element retaining flange or threaded boss, clean the sealing surface, fit a new Oso‑compatible gasket and element, and torque to manufacturer spec. Refill, bleed air from hot taps, and check for leaks.
• Restore power and test: Once the cylinder is full and leak‑free, restore power and confirm the E01 code clears and the heater begins to warm.

If the code persists after replacing the element, the control board or wiring harness may be defective and should be checked by a qualified technician.