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Boiler PCB (main control board): What It Does and Signs It's Failing

The PCB is the boiler's brain, a circuit board in a plastic housing behind the front controls. It reads every sensor, runs the fan, pump, diverter and spark, and opens the gas valve. The display and buttons on the front are often a separate board linked to it, and a wiring harness connects everything together.

How it works

The board follows a fixed start sequence: check pressure and sensors, start the fan, spark, open the gas, prove the flame. If any step fails it stops and shows a code. Damp, heat and loose or corroded harness plugs are common reasons boards and connections play up.

Signs it is failing

• Blank display with power reaching the boiler
• Random codes that change or make no sense
• Boiler that won't respond to its own buttons or the thermostat
• Burnt smell or scorch marks near the controls
• Faults that clear when the boiler is tapped or warms up

Vaillant lists the PCB for several ignition, flame and gas valve control codes, and the harness for sensor and fan faults. An invalid-device code usually appears right after a board or display swap.

Can you replace it yourself?

Board, display and harness work happens inside the casing on mains voltage, and the new board often needs setting up for the exact model, so it's for a Gas Safe engineer or RGI. Your checks: the fused spur switch is on, any timer or thermostat batteries are fresh, and one power-off reset if your manual allows it. If the boiler's power cable is damaged, don't replace it yourself.

Buying the right part

Boards are model-specific and sometimes need a code entered after fitting. Genuine parts are worth paying for; Ideal says not to use reconditioned or copy parts it hasn't authorized.

Error codes linked to the pcb (main control board)

• E133Gas supply interrupted or flame failed to establishBaxi
• F.28Boiler failed to lightVaillant
• F.25Flue gas temperature too highVaillant
• F.27Flame detected with the burner offVaillant
• F.00Flow or return temperature sensor not readingVaillant
• F.10Temperature sensor short circuitVaillant
• F.13Hot water cylinder sensor short circuitVaillant
• F.32Fan speed out of rangeVaillant
• F.61Gas valve control faultVaillant
• F.62Flame took too long to go outVaillant
• F.70Invalid device code after a repairVaillant
• EA / EA 227Burner didn't light (no flame detected)Worcester Bosch
• E9 / E9 224Overheat limiter trippedWorcester Bosch
• FdReset button pressed or power cut during a faultWorcester Bosch
• C6Fan running too slowlyWorcester Bosch
• E2Heating flow temperature sensor faultWorcester Bosch
• A7Hot water temperature sensor faultWorcester Bosch
• A8Lost contact with the controlsWorcester Bosch
• F7Flame signal with the burner offWorcester Bosch
• FAFlame still present after the gas shut offWorcester Bosch
• F0Internal control faultWorcester Bosch
• b1Code plug not detectedWorcester Bosch
• CCOutside weather sensor not readingWorcester Bosch

Last reviewed 2026-10-21.