Sime AL 83 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 83 fault code mean?
AL 83 on a Sime boiler flags irregular combustion as a temporary error: the combustion-control system saw a burn deviate from what it expects, logged it, and — unlike a lockout — generally carries on or retries. Sime's documented check for it is a blockage in the flue. AL 83 is best understood as an early warning: something disturbed combustion (a gust down a marginal flue, a transient in gas pressure, an electrode ageing toward the edge of acceptability), and while one instance is tolerable, a pattern is not — repeated AL 83 events escalate to the AL 82 block. A boiler logging AL 83 more than very occasionally is asking for a service visit before it locks out in midwinter.
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
- Transient flue disturbance Common
Wind effects on a marginally sited terminal, or the early stages of an obstruction, briefly upset the air side of combustion. Sime's check for AL 83 is the flue for precisely this reason.
- Gas supply transients Sometimes
Momentary pressure dips — heavy simultaneous demand, supply-side issues — disturb single burns and log the code without recurring immediately.
- Electrode ageing Sometimes
A wearing electrode produces occasional marginal flame feedback long before it fails outright; scattered AL 83 events are often its first symptom.
How to fix it
- Note the frequency DIY safe
A one-off AL 83 after a storm is unremarkable. Weekly or daily occurrences are a trend — book a service before it escalates to an AL 82 lockout.
- Glance at the flue terminal DIY safe
Check the outside terminal for developing obstructions — vegetation, nests, winter ice. Visual check only.
- 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 £90–£300, depending on the underlying cause.
Frequently asked questions
What does AL 83 mean on a Sime boiler?
The combustion controller logged one irregular burn — a temporary deviation, usually flue-related, occasionally gas-supply or electrode ageing. It is a warning rather than a lockout, but repeated instances escalate to the AL 82 block, so treat a pattern as a prompt to book a service.
Do I need to do anything about a single AL 83?
Note it and carry on. If it recurs, especially in similar conditions (windy days, certain times), tell an engineer — the pattern is diagnostic, and catching a marginal flue or ageing electrode early is far cheaper than a winter lockout.
Why does Sime point at the flue?
Because combustion quality on these boilers is actively controlled, the commonest external disturbance is the air path — and a partly blocked or wind-affected flue is the usual way that path degrades. The engineer's checks then widen to gas pressure and the electrode.