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Boiler Gas valve and injector: What It Does and Signs It's Failing

The gas valve is the electrically controlled block where the gas pipe enters the boiler. It opens to let gas through only when the board calls for heat, and its built-in governor sets how much. Gas then passes through an injector, a small brass jet, into the fan and venturi where it mixes with air before reaching the burner.

How it works

The valve only stays open while the board powers it. Cut that power and it closes, which is how the boiler's safety devices stop the gas. Modern valves are zero-pressure governor types that follow the fan, so more air means more gas.

Signs it is failing

• Boiler sparks but never lights while other gas appliances work fine
• Flame that won't go out promptly when the boiler switches off
• Unstable flame or burner cutting out under load
• Gas valve fault code on the display
• Smell of gas near the boiler (leave the building and call your gas emergency number)

Ignition, flame-loss, gas valve control and slow flame-out codes involve the gas valve. Vaillant names the injector for an unstable flame.

Can you replace it yourself?

Never touch, adjust or take apart the gas valve or injector; in the UK it's illegal for anyone but a Gas Safe registered engineer, and in Ireland only a Registered Gas Installer (RGI) may work on it. You can check that gas reaches your hob, that the meter has credit and that the boiler's gas cock is open. If you smell gas, open windows, don't use switches, turn off at the meter if it's safe, and call your gas emergency number from outside.

Buying the right part

Gas valves and injectors are set for natural gas or LPG and for a particular output, so the engineer will fit only the part listed for your model and gas type.

Error codes linked to the gas valve and injector

• 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.61Gas valve control faultVaillant
• F.62Flame took too long to go outVaillant
• F.68Unstable flameVaillant
• EA / EA 227Burner didn't light (no flame detected)Worcester Bosch
• FAFlame still present after the gas shut offWorcester Bosch

Last reviewed 2026-10-21.