Your boiler's internal computer has failed a self-test and is no longer able to communicate with the rest of the system.

Fault: Main PCB self-test failed. Main PCB defective

What causes this fault?

This fault occurs when the boiler's central control board fails its internal safety checks during startup or operation. The most common reason is a hardware malfunction within the circuitry, often caused by a component reaching the end of its lifespan or suffering damage from an electrical power surge. Because the board can no longer safely manage the gas and ignition process, it shuts the system down to prevent any hazards.