Sime AL 42 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 42 fault code mean?
AL 42 on a Sime boiler is a button fault: the control board has detected that one of the panel buttons is misbehaving — typically reading as stuck, pressed continuously or electrically inconsistent. Because the buttons feed directly into the control logic (a jammed reset or mode key could mask real faults or force unwanted states), the board flags the condition rather than ignore it. Sime's terse remedy is to check that the buttons are working. In practice the culprit is a sticking key on the panel, moisture or debris under the membrane, or a fault in the panel electronics; the cure ranges from freeing a stuck key to replacing the control panel or board.
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
- Stuck or sticking button Common
A key held down by debris, a misaligned fascia or a worn membrane reads as continuously pressed and trips the check. Sometimes visibly obvious — the key sits lower or does not click back.
- Moisture or contamination in the panel Sometimes
Condensation or a cleaning spray working under the membrane can bridge button contacts intermittently, producing phantom presses.
- Panel or board electronics fault Sometimes
If every key is free and dry and AL 42 persists, the button-reading circuitry on the panel or PCB has failed and the part needs replacing.
How to fix it
- Look at and gently press each button DIY safe
Check whether any key sits lower than its neighbours, feels sticky, or fails to spring back. Press each once firmly to free a marginal key. Do not prise at the fascia or use liquids.
- 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
If the fault persists, an engineer will test the button inputs, and replace the control panel or board as required — the panel is inside the appliance casing, so this is not a DIY repair despite the fault sounding trivial.
Parts you may need
- Control panel/fascia assembly · from £90
- 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–£260, depending on the underlying cause.
Frequently asked questions
What does AL 42 mean on my Sime boiler?
One of the panel buttons is reading as stuck or faulty. The board flags it because a jammed key could interfere with resets and mode changes. Often it is literally a sticking button you can free with a firm press; if not, the panel electronics need professional attention.
The boiler still heats with AL 42 showing — can I ignore it?
Don't ignore it for long. Even if heating continues, a faulty button can block you from resetting a future fault or changing settings when you need to. Free the key if you can, and book an engineer if the code stays.
What will it cost?
Nothing if a stuck key frees; a modest amount for a replacement panel; more only if the main board is behind it — typically £80–£260 all-in depending on which. Get a written quote.