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Boiler Flame sensor (flame detection electrode): What It Does and Signs It's Failing

The flame sensor is a metal rod held in a ceramic insulator so its tip sits in the burner flame. It proves to the control board that there's a flame while the gas valve is open. Worcester's Greenstar i uses a separate rod, while Ideal's Logic Combi2 C has one combined electrode that sparks and senses.

How it works

The board puts a voltage on the rod, and the flame itself carries a tiny current between the rod and the burner. That signal (engineers call it ionization or flame rectification) is the board's proof of flame. If it doesn't show up, or drops out while the boiler runs, the board shuts the gas valve and tries again, then locks out after repeated failures.

Signs it is failing

• Burner lights for a few seconds, then goes out
• Flame-loss or flame-failure lockout
• Boiler cutting out partway through heating or a shower
• Flame-detected-with-burner-off code

Failed-to-light, flame-loss, unstable-flame and false flame codes all bring this electrode into question. A flue or condensate blockage can produce the same symptoms, so the engineer checks those too.

Can you replace it yourself?

This sits inside the combustion chamber and is checked by a Gas Safe engineer (an RGI in Ireland) at service time. Don't remove the casing to clean it. Your checks stop at confirming the gas supply, looking at the condensate pipe outside in frosty weather, and one reset.

Buying the right part

The rod's length and bend set where it sits in the flame, so only the part listed for your boiler model will do. Replace the seal at the same time.

Error codes linked to the flame sensor (flame detection electrode)

• E133Gas supply interrupted or flame failed to establishBaxi
• E128Flame failureBaxi
• F.28Boiler failed to lightVaillant
• F.29Flame went out and the boiler couldn't relightVaillant
• F.27Flame detected with the burner offVaillant
• F.68Unstable flameVaillant
• EA / EA 227Burner didn't light (no flame detected)Worcester Bosch
• F7Flame signal with the burner offWorcester Bosch

Last reviewed 2026-10-21.