Boiler does not start at all; no display, no lights, and no response to controls.

Safety first: Before opening the boiler, turn off the breaker and verify absence of voltage with a suitable tester if you are qualified.

• Check external power: Verify that the dedicated boiler breaker is ON and not tripped. Reset if necessary.
• Inspect power outlet or spur: Ensure any fused spur or plug supplying the boiler has an intact fuse and good connections.
• Examine internal fuse: With power off, remove the front cover and locate the internal fuse on the control PCB. If blown, replace only with the exact type and rating specified by Rinnai. If it blows again, stop and call a technician.
• Check power wiring: Confirm that live, neutral, and earth wires are securely connected at the boiler terminal block.
• PCB replacement: If correct power is present at the board but the display remains dead, the main PCB likely needs professional replacement.