Boiler does not ignite, display shows E01, no flame symbol, heating and hot water unavailable

Safety first: Turn the boiler off at the control panel and isolate electrical power at the fused spur. Close the gas isolation valve before removing any covers. Only Gas Safe–registered engineers should work on gas-carrying components.

• Reset the boiler: Press the reset button as per the Sime user manual and wait a full ignition cycle. If E01 returns, continue below.
• Check gas supply: Ensure other gas appliances work. Verify the boiler's gas isolation valve is fully open. If there is no gas supply, contact your gas supplier.
• Inspect flue terminal: From outside, confirm the flue is not blocked by debris, snow, or nests, which can affect combustion and flame sensing.
• Check ignition lead and electrode (engineer task): Remove the front case, visually inspect the ignition electrode and lead for cracks, carbon build-up, or loose connections. Clean the electrode with fine emery paper and re‑seat the lead firmly.
• Check flame sensor (engineer task): On many Sime boilers the flame sensor is combined with the ignition electrode. Confirm correct gap and position relative to the burner. Replace if the ceramic is cracked or heavily worn.
• Gas valve and PCB checks (engineer task): Using a multimeter and manometer, the engineer should verify that the gas valve is being powered during ignition and that burner pressure is within Sime specifications. If the valve is powered but not opening, replace the gas valve. If no power is sent, suspect the main PCB.

Do not attempt to adjust gas valve settings or work on sealed combustion components unless you are a qualified engineer.