Oso water heater displays E05 and will not operate; may be associated with a recent power surge or wiring work.

Safety first: Isolate the heater at the breaker and verify no voltage is present before opening the control compartment.

• Power cycle the unit: Turn off the breaker for at least 5 minutes to allow capacitors to discharge, then restore power and see if E05 clears.
• Inspect control wiring: Remove the front cover and check all low‑voltage connectors between the display module, sensor harness, and main control PCB. Reseat each connector firmly.
• Check for moisture or corrosion: Look for signs of water ingress in the control area. If present, identify and repair the leak (e.g., from T&P valve or pipework) and dry the electronics thoroughly before re‑energizing.
• Verify supply voltage: Use a multimeter to confirm that the incoming supply to the control board matches the rating plate (e.g., 230 V). Incorrect voltage can cause control faults.
• Replace the PCB if needed: If E05 persists and all external components test good, the main control PCB is likely defective. Order the correct Oso replacement board, transfer any configurable jumpers or settings as per the service manual, and install carefully.

Because this involves live mains and control electronics, board replacement should be carried out by a qualified technician.