Sime AL 92 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 92 fault code mean?
AL 92 on a Sime boiler means the combustion controller has reached the maximum air correction available (at the minimum flow rate) — the closed-loop system has trimmed the air side of the mixture as far as its authority allows and still cannot bring combustion to target at low output. Running out of correction range is diagnostic in itself: something has shifted the combustion so far from baseline that the trim cannot bridge it. Sime's checklist is the standard combustion set — ignition/detection electrode, flue outlets, air diaphragm, gas calibration — since a worn electrode, a restricted air path, a degraded diaphragm or a drifted baseline are the four ways a boiler drifts to the edge of its correction authority.
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-path restriction at low rate Common
Minimum-output combustion is the most sensitive to air-side problems; a partial flue or intake restriction consumes correction authority fastest exactly there.
- Air diaphragm degradation Common
The diaphragm shapes the air metering the trim works against; as it ages the controller compensates until it hits the limit — AL 92 is that limit.
- Electrode drift skewing the feedback Sometimes
A degraded electrode misreports combustion quality, driving the controller to over-correct until authority runs out.
- Baseline drift needing recalibration Sometimes
If the stored calibration no longer matches the hardware, corrections start from the wrong place; recalibration restores headroom once physical causes are fixed.
How to fix it
- Check the flue terminal externally DIY safe
A visual check for obstruction, especially in winter. No other user-side action exists for this code.
- 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–£380, depending on the underlying cause.
Frequently asked questions
What does AL 92 mean on my Sime boiler?
The combustion controller has used all its air-correction range at minimum output and still cannot hit its combustion target — so it stops. It means combustion has drifted a long way from baseline: typically an air-path restriction, an ageing diaphragm or electrode, or a calibration that no longer fits. An engineer works Sime's checklist and recalibrates.
Why does the manual mention minimum flow rate?
Low-output combustion is where the mixture is most delicate and where air-side problems bite first — the controller's correction authority is exhausted there before it fails at full rate. That is why the fault names the minimum-rate condition.
Will it come back after repair?
Not if both halves are done: fix the physical cause and rerun the self-calibration so the controller starts from a true baseline with full correction authority restored. Skipping the recalibration invites a repeat.