Sime AL 90 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 90 fault code mean?
AL 90 on a Sime boiler means the combustion controller cannot reach its combustion set point: the closed-loop system that trims the gas/air mixture has driven its correction as directed, but the measured combustion never arrived at the target. The boiler stops because a controller that cannot reach its set point is either working against a physical obstruction (flue restriction, gas starvation), reading through a degraded sensor (electrode), or trimming against a baseline that no longer matches the hardware (calibration). Sime's remedy list mirrors this: electrode, flue outlets, air diaphragm, gas calibration — with the automatic self-calibrating procedure rerun once the physical cause is corrected. AL 93 is a closely related variant of the same failure.
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
- Flue or air-path restriction Common
If the air side cannot deliver, no amount of trimming reaches the target. Partial blockages at the terminal or in the ducts are a leading cause.
- Electrode degradation distorting feedback Common
The controller steers by the electrode's signal; a worn electrode reports combustion that never seems to reach target even when the burn is close. New electrode, recalibrate, resolved.
- Gas supply weakness Sometimes
Low working pressure or air in the gas circuit caps what the mixture can achieve — the gas side simply cannot meet the set point (compare AL 84).
- Stale calibration baseline Sometimes
After component changes without recalibration, the target itself is wrong for the hardware. Rerunning the self-calibration (AL 62's remedy) is part of the standard fix.
How to fix it
- Check the flue terminal externally DIY safe
Look for obstruction — the only user-safe check on a combustion-reach fault.
- 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 90 mean on a Sime boiler?
The self-regulating combustion system tried to steer the burn to its target and could not get there, so the boiler stopped. The blockers are physical (flue restriction, weak gas supply), sensory (worn electrode) or calibration (stale baseline) — Sime's checklist covers all four, finishing with a rerun of the self-calibration.
Is AL 90 the same as AL 93?
They are variants of the same 'combustion set cannot be reached' failure, distinguished internally by the controller. For diagnosis and repair they are treated identically — same checklist, same recalibration afterwards.
Can cold weather trigger this?
Indirectly, yes — winter brings iced terminals (air-path restriction) and peak gas demand (supply pressure dips), both of which can stop the set point being reached. The engineer's working-pressure and flue checks catch each.