Sime AL 88 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 88 fault code mean?
AL 88 on a Sime boiler is an internal error raised by the control board's own protection — a component-level fault detected within the board itself rather than in any external sensor or actuator. Sime's manual is unusually blunt about the remedy: check that the board is working, and replace the board. Modern combustion-control boards continuously self-test their critical circuits; when one of those self-tests fails, the board declares itself unfit and stops the boiler, because every safety judgement it makes depends on its own integrity. An engineer will confirm the diagnosis (ruling out supply and connection issues that can occasionally masquerade as internal faults) and then replace and recommission the PCB.
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
- Control board component failure Common
The board's self-test has caught a failed internal component. Sime's own remedy is board replacement — this is the expected diagnosis for AL 88.
- Supply disturbance corrupting the board state Sometimes
A severe spike or brownout can occasionally trip internal protection without permanent damage; a full power-cycle distinguishes a latched state from a genuine failure.
- Moisture or connection damage on the board Rare
Leaks reaching the control enclosure or corroded multi-pin connectors can fail internal checks; found on inspection before a replacement is fitted.
How to fix it
- Power-cycle once at the fused spur DIY safe
Switch off for a full minute and back on — the one safe test that separates a transient latched fault from a genuine board failure. If AL 88 returns, the board needs professional attention.
- 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 an engineer confirm and replace the board Gas Safe engineer
The engineer will inspect for moisture and connector damage, verify supply quality, and — per Sime's documented remedy — replace the PCB. On these models board replacement is followed by the full parameter set-up, automatic self-calibration and a flue-gas analysis before handover.
Parts you may need
- Control PCB · from £220
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 £200–£450, depending on the underlying cause.
Frequently asked questions
What does AL 88 mean on a Sime boiler?
The control board's own self-tests have failed — an internal component-protection error. Sime's manual goes straight to checking and replacing the board. Try one power-cycle to rule out a transient; if the code returns, it is a PCB replacement and recommission.
Is a board replacement worth it on an older boiler?
It is usually the most expensive single repair (typically £200–£450 fitted, including the mandatory recalibration). On a boiler past 10–12 years, weigh it against replacement — an honest engineer will give you both numbers. We do not sell boilers, so judge on the quotes.
Could anything else cause AL 88?
Occasionally a severe supply disturbance latches the protection, and rarely moisture or connector damage is found on inspection — which is why the engineer checks those before condemning the board. But the manual's expectation, and the usual outcome, is the board itself.