Sime AL 12 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 12 fault code mean?
AL 12 on a Sime boiler means the control board's combustion-chamber configuration does not match what the boiler expects — Sime's manual describes it as an incorrect configuration of the open/sealed chamber. The board holds a parameter (tS 0.4, the combustion configuration) that tells it whether it is running as a sealed room-sealed appliance with combustion control, an open-chamber appliance with a smoke thermostat, or in Low NOx mode. If that parameter is wrong — almost always after a control board has been replaced and set up incompletely — the boiler refuses to run, because operating a sealed-chamber boiler with open-chamber logic (or vice versa) would undermine its combustion safety strategy. The documented fix is simply to set parameter tS 0.4 to the correct value (0 for the sealed-chamber UK models) and recommission.
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
- Combustion configuration parameter set wrongly after a PCB change Common
When a replacement board is fitted, the installer must set the parameter table for the model, including tS 0.4. Missing or mis-setting that step leaves the board configured for the wrong chamber type, and AL 12 appears as soon as it runs its checks.
- Parameters corrupted or reset to defaults Sometimes
A board fault or a parameter reset (tS 4.8) can return settings to defaults that do not match the appliance, tripping the configuration check.
- Control board fault Rare
Rarely the parameter is correct but the board misreads its own configuration; if AL 12 persists after correct set-up, the PCB itself is suspect.
How to fix it
- Note whether the boiler has just had work done DIY safe
AL 12 almost always follows a control board replacement or parameter change. If an engineer recently worked on the boiler, contact them — completing the parameter set-up is part of that job.
- Reset once with the OK/RESET button DIY safe
Hold the OK RESET button for more than 3 seconds. Sime boilers allow a limited number of consecutive resets (six) before locking out on AL 28, so if the code returns after one or two attempts, stop and book an engineer rather than using them all up.
- Have a Gas Safe registered engineer carry out Sime's documented remedy Gas Safe engineer
The engineer will enter the installer parameter menu and set tS 0.4 (combustion configuration) to the correct value for the model — 0 for the sealed-chamber UK models — then run the automatic self-calibrating procedure and verify combustion with a flue gas analysis, as Sime's manual requires after board or parameter changes.
Parts you may need
- Control PCB · from £200
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 £80–£250, depending on the underlying cause.
Frequently asked questions
What does AL 12 mean on my Sime boiler?
The board's combustion-chamber configuration parameter (tS 0.4) does not match the appliance — it thinks it is the wrong type of boiler (open versus sealed chamber). It nearly always appears after a control board has been replaced without the parameters being fully set. An engineer corrects the parameter and recommissions; it is configuration, not component failure.
Can I change parameter tS 0.4 myself?
No. The installer parameter menu changes how the boiler manages combustion, and errors there have safety consequences. Setting it, and the mandatory self-calibration and flue-gas analysis afterwards, is Gas Safe registered engineer work.
Is AL 12 expensive to fix?
Usually not — if it is just the parameter, it is a short visit to configure and recommission. Only if the board itself proves faulty does the cost rise toward the top of the typical £80–£250 range.