Sime AL 41 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 41 fault code mean?
The AL 41 code appears on Sime boilers when the burner flame has been lost six times in a row during the ignition sequence. After this sixth consecutive failed attempt to establish and hold a flame, the boiler locks out entirely and displays AL 41 to protect itself from further damage. This is a safety lockout — the boiler will not restart automatically and requires manual intervention. The root cause can range from a straightforward gas supply issue to a faulty component within the ignition or burner system, so the fault needs to be investigated carefully before simply resetting the boiler repeatedly.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Interrupted or low gas supply Common
If the gas supply reaching the boiler is insufficient, intermittent, or has been disrupted — for example, due to a closed isolation valve, low pressure at the meter, or an empty LPG cylinder — the burner cannot sustain a flame and will cut out repeatedly until lockout occurs. This is the first thing to rule out.
- Faulty ignition electrode or lead Common
The ignition electrode produces the spark that lights the burner. Over time the tip can wear, crack, or become coated with carbon deposits, leading to a weak or absent spark. Similarly, the high-tension lead connecting the electrode to the spark generator can degrade and fail to carry the ignition pulse reliably.
- Flame sensor (ionisation probe) failure Common
Once the burner lights, a flame sensor detects the presence of the flame by measuring a small electrical current through the flame itself. If the probe is dirty, corroded, or has failed, the boiler's control board cannot confirm the flame is present and shuts the gas off — triggering repeated lockouts even if ignition was briefly successful.
- Faulty or sticking gas valve Sometimes
The gas valve controls the flow of gas to the burner. If it fails to open fully, sticks, or opens erratically, gas delivery to the burner will be inconsistent and the flame will drop out. Gas valve faults require a Gas Safe engineer to diagnose and replace.
- Blocked or dirty burner Sometimes
A build-up of debris, scale, or corrosion on the burner head can disrupt the gas-air mixture required for stable combustion. A partially blocked burner may briefly ignite but fail to sustain a proper flame, causing repeated drop-outs and eventually triggering AL 41.
- Blocked or restricted flue Sometimes
The flue carries combustion gases safely out of the property. If it is obstructed — by a bird's nest, debris, or an incorrectly installed terminal — combustion products can back up into the heat exchanger, starving the flame of oxygen and causing it to extinguish. Check the external flue terminal is clear.
- PCB (printed circuit board) fault Rare
In rarer cases the main control board may be misreading signals from the flame sensor or ignition circuit, triggering a false AL 41 lockout even when the ignition components themselves are serviceable. PCB diagnosis requires specialist equipment and must be carried out by a Gas Safe engineer.
How to fix it
- Check your gas supply is active DIY safe
Before anything else, confirm that other gas appliances in the property — such as a gas hob or gas fire — are working normally. If they are not, the fault is with your gas supply rather than the boiler itself: contact your gas supplier. Also check that the boiler's gas isolation valve (the handle on the pipe leading into the boiler) is in the open position — the handle should be in line with the pipe, not across it.
- Reset the boiler DIY safe
Once you have confirmed the gas supply is on, press and hold the reset button (refer to your Sime boiler's user manual for the exact location) until the lockout indicator clears, then allow the boiler to attempt ignition. If it lights and runs normally, monitor it over the next hour or so. Do not reset the boiler more than two or three times in quick succession — repeated resets without finding the underlying cause can cause further damage and will not resolve the fault.
- Check the external flue terminal for obvious blockages DIY safe
Go outside and visually inspect the flue outlet on the external wall. Look for bird nests, leaves, debris, or any build-up of ice in cold weather that could be restricting the exit of flue gases. Do not insert your hand or any tool into the flue. If you can see a clear obstruction at the terminal end and can safely remove it without touching the flue pipework itself, do so, then attempt a reset. If you are unsure or cannot safely access it, leave this to an engineer.
- Contact a Gas Safe registered engineer to inspect the ignition system Gas Safe engineer
If the boiler continues to lock out with AL 41 after the above checks, the fault is almost certainly within a component that only a Gas Safe registered engineer may legally inspect, test, or replace. The engineer will measure gas inlet pressure, test the spark at the electrode, check the ionisation current from the flame sensor, and assess the condition of the gas valve. Do not attempt to access or adjust any of these components yourself.
- Engineer to inspect and clean or replace the burner Gas Safe engineer
As part of the investigation, the engineer should inspect the burner for deposits, corrosion, or damage that could be disrupting combustion. A burner service or replacement may resolve the fault without needing to replace more expensive parts.
- Engineer to test and replace faulty components as needed Gas Safe engineer
Depending on diagnosis, the engineer may replace the ignition electrode and lead, the flame sensing probe, or the gas valve. In rare cases the PCB may need to be tested and replaced. Only parts and labour carried out by a Gas Safe engineer will be covered by any remaining manufacturer warranty, and this work is a legal requirement under Gas Safety (Installation and Use) Regulations 1998.
Parts you may need
- Ignition electrode and lead set · from £45
- Flame sensor / ionisation probe · from £35
- Gas valve · from £130
- Burner assembly · from £95
- Spark generator / ignition transformer · from £55
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £120–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Sime AL 41 myself by resetting the boiler?
A single reset is worth trying once you have confirmed your gas supply is on and the flue terminal is unobstructed. However, if the boiler locks out again after one or two resets, resetting it further will not cure the underlying fault and may mask a safety issue. The AL 41 lockout specifically indicates that the flame failed to establish six times in a row, which points to a component fault — such as a worn electrode, a faulty flame sensor, or a gas valve problem — that requires a Gas Safe registered engineer to diagnose and repair legally.
How much does it cost to repair a Sime AL 41 fault in the UK?
For most households, the repair falls between £120 and £380, depending on which component has failed. Replacing an ignition electrode and lead is typically at the lower end (roughly £120–£180 including labour), while a gas valve replacement usually sits between £200 and £350 all in. If a PCB is at fault, costs can exceed £400–£500, though this is less common. Always obtain at least two quotes from Gas Safe registered engineers, and ask whether the diagnosis call-out fee is included in the repair price.
Why does my Sime boiler keep showing AL 41 every few days?
An intermittent or recurring AL 41 fault usually points to a component that is degrading rather than completely failed — most often a flame sensor that is becoming dirty or an electrode that is starting to wear. The boiler ignites successfully most of the time but drops the flame occasionally, eventually accumulating six consecutive failures and locking out. This pattern tends to worsen over time rather than self-resolve. Have a Gas Safe engineer inspect the ignition and sensing components sooner rather than later, as an intermittent fault is easier and cheaper to address before it becomes a complete failure.
Could a blocked condensate pipe cause AL 41 on a Sime boiler?
It is unlikely but not impossible. A blocked condensate pipe — which commonly freezes in very cold weather — causes most Sime boilers to display a different fault code related to condensate or water pressure. However, if a severe blockage causes the boiler to shut down abnormally mid-cycle, a flame-loss code can occasionally follow. If you notice the fault during a cold snap, check whether the condensate pipe (the plastic pipe that runs from the boiler to an external drain) is frozen and thaw it gently with warm (not boiling) water. If the boiler then restarts and runs normally, condensate was the culprit. If AL 41 persists, the cause lies elsewhere and an engineer is needed.