Oso water heater not heating at all, no error code displayed; water remains cold.

Safety first: Turn OFF power at the breaker and verify isolation before opening any covers.

• Check external power: Verify the breaker or fused spur supplying the Oso heater is ON and the fuse is intact. Reset or replace only once; if it trips again, investigate further.
• Inspect terminal block: Remove the front cover and check that live and neutral are present at the heater’s terminal block (test with a multimeter if competent).
• Test thermostat: With power off, check continuity across the control thermostat. If open when set above ambient, replace it.
• Test element: Disconnect element wires and measure resistance. Replace the element if open or shorted.

If you are not comfortable working with mains electricity, contact a qualified electrician or service technician.