Sime AL 81 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 81 fault code mean?
AL 81 on a Sime boiler is a block caused by combustion problems during start-up: the combustion-control system watched the burner light and immediately judged the combustion it saw to be unacceptable, so it shut the attempt down. Sime's documented checks are a blockage in the flue and air trapped in the gas circuit — the two things that most reliably wreck the first seconds of combustion. A restricted flue starves or destabilises the flame; air in the gas line (typically after work on the gas supply or a long shutdown) means the first gas to reach the burner is not gas at all. Repeated AL 81 lockouts merit prompt professional attention rather than repeated resets.
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
- Air in the gas circuit Common
After gas supply work, a meter change or a long shutdown, the gas line can hold air. First light-offs sputter and fail combustion checks until the air is purged — Sime's manual explicitly lists bleeding the air from the gas circuit for this code.
- Flue inlet or outlet blockage Common
Debris, nests or ice restricting the flue disturb the air/gas balance most severely at ignition, when combustion is being established. Sime's first check for AL 81 is the flue.
- Electrode or combustion-feedback deterioration Sometimes
A worn ignition/detection electrode gives the combustion controller a poor view of the start-up flame, failing starts that would otherwise pass. Usually accompanied by other combustion-family codes over time.
How to fix it
- Check the flue terminal visually from outside DIY safe
Look for obvious blockage at the terminal — leaves, a nest, ice in winter. Look only; never reach into or dismantle the flue.
- Consider whether gas work happened recently DIY safe
If the meter, supply or boiler gas line was recently worked on, air in the gas circuit is likely — the engineer purges it safely. Do not attempt to purge gas lines yourself.
- Reset once with the OK/RESET button DIY safe
Hold the OK RESET button for more than 3 seconds. Sime allows a maximum of six consecutive resets before locking out on AL 28, so if the code returns after one or two attempts, stop and book an engineer.
- Have a Gas Safe registered engineer work through the combustion-control checks Gas Safe engineer
Sime's documented checklist for the combustion-control faults covers the ignition/detection electrode (condition, position and earthing), the flue inlet and outlet for blockage, the air diaphragm, the gas calibration and supply pressure, and bleeding air from the gas circuit where relevant. After rectification the engineer reruns the automatic self-calibrating procedure and confirms combustion with a flue-gas analysis.
Parts you may need
- Ignition/detection electrode · from £35
- Air diaphragm · from £15
- Gas valve (SGV) · from £150
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 £100–£350, depending on the underlying cause.
Frequently asked questions
What does AL 81 mean on a Sime boiler?
The combustion-control system rejected the combustion it saw during start-up and blocked the boiler. Sime's own checklist is a blocked flue and air in the gas circuit — the two classic ways a start goes bad — with the electrode as the follow-up suspect. One reset is fine; repeats need an engineer.
The boiler was fine until my gas meter was changed — related?
Very possibly. Supply work introduces air into the gas line, and the first ignition attempts afterwards can fail combustion checks exactly like this. An engineer purges the circuit and the boiler typically returns to normal.
Can I bleed the gas circuit myself?
No — purging air from gas pipework is gas work under UK regulations and must be done by a Gas Safe registered engineer. Your safe contributions are the visual flue check and a single reset.