Sime AL 28 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 28 fault code mean?
The AL 28 code is Sime's way of signalling a serious internal or electronic fault that goes beyond routine diagnostics. When the boiler's control board detects a problem it cannot resolve through normal operation, it locks out the appliance and displays AL 28 to alert you that specialist technical attention is needed. In practice, this code almost always points to a fault with the PCB (Printed Circuit Board) — the central control unit that monitors every component, manages ignition sequencing, and enforces safety shutdowns. Because the board sits at the heart of the boiler, a failure here can affect heating, hot water, and every safety check the boiler performs. This is not a code you can reset your way out of; professional diagnosis is essential.
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
- PCB failure due to component ageing Common
After many years of continuous heating cycles, the internal circuits, solder joints, and connectors on the PCB gradually deteriorate. Older boards become increasingly prone to random lockouts and error codes as tiny fractures or dry joints develop across the circuitry. This is the most common reason AL 28 appears on boilers that are a decade or more old.
- Water or moisture ingress onto the PCB Common
A slow internal leak — from a failing pump seal, heat exchanger pinhole, or excessive condensation inside the casing — can allow moisture to reach the PCB. Even a small amount of water on an energised board can cause corrosion, short circuits, or permanently damaged components, triggering an AL 28 lockout.
- Loose wiring or connector issues Sometimes
Vibration from the pump, fan, or general boiler operation can gradually work connectors loose or cause wire insulation to chafe against metal edges. A poor connection to or from the PCB can produce erratic behaviour that the board interprets as an internal fault, leading to AL 28 being displayed.
- Burnt or secondary-damaged board components Sometimes
Sometimes the PCB itself is not the original source of the problem. A failing external component — such as an overheating pump or a short-circuiting sensor — can send an incorrect signal or voltage spike to the board, burning individual components. In these cases, the underlying fault must be identified and fixed before or alongside any PCB work.
- Manufacturing defect or installation fault Rare
On a small number of units, a PCB fault can present early in the boiler's life due to a manufacturing defect, poor-quality component, or an error during initial installation. This is uncommon but worth noting — Sime's Format C range (models 80C, 100C, and 110C) was the subject of a historical Gas Safe safety alert relating to a PCB fault that caused overheating in a very small proportion of units.
How to fix it
- Attempt a single boiler reset DIY safe
Locate the reset button on your Sime boiler — usually marked with a flame symbol or a dedicated reset label — and hold it for a few seconds. If the boiler fires up normally, monitor it closely over the next hour. If AL 28 returns, do not reset again. Repeated resets when a PCB fault is present can put additional stress on the board and will not clear the underlying problem.
- Check that your gas supply is active DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are, your gas supply is not the issue. If nothing gas-powered is working, contact your gas supplier before calling an engineer.
- Contact a Gas Safe registered engineer for diagnosis Gas Safe engineer
AL 28 requires hands-on inspection of the boiler's internal electronics by a qualified professional. The engineer will open the casing, inspect the PCB and all connected wiring, and check for signs of water damage, burnt components, or loose connectors. Do not attempt to open the boiler or touch internal components yourself.
- Engineer checks for underlying faults before addressing the PCB Gas Safe engineer
A good engineer will not simply replace the PCB and walk away. They will first confirm there are no leaks, failing pumps, or faulty sensors that may have caused or contributed to the board fault. Replacing a PCB without fixing the root cause risks damaging the new board in the same way.
- PCB repair or replacement if confirmed faulty Gas Safe engineer
If the board is confirmed beyond saving, the engineer will source a compatible replacement PCB for your specific Sime model — this often needs to be ordered directly, so allow a day or two for the part to arrive. Fitting a new PCB typically takes a couple of hours and requires careful commissioning to confirm all boiler functions are restored correctly.
Parts you may need
- Sime replacement PCB (control board) · from £280
- Wiring harness / connector loom · from £45
- Pump (if moisture ingress source) · from £95
- Pressure sensor / transducer · from £35
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 £320–£600, depending on the underlying cause.
Frequently asked questions
Can I fix the AL 28 fault myself?
No. AL 28 points to an internal electronic or PCB-level fault, and any work on the boiler's internal components must be carried out by a Gas Safe registered engineer. Attempting to open the boiler or interfere with the wiring yourself is both dangerous and illegal under UK gas safety regulations. The one thing you can safely try is a single reset — but if the code comes back, call an engineer.
How much does it cost to fix a Sime AL 28 fault in the UK?
Most homeowners with this fault pay somewhere between £320 and £600 for a full repair, including parts and labour. The PCB itself typically costs £180–£350 depending on the exact Sime model, with engineer time on top. In some cases — for instance where a leak has damaged the board and additional components need replacing — costs can push higher. If your boiler is over 12–15 years old and the repair quote exceeds roughly half the cost of a new boiler, it is worth getting a replacement boiler quote for comparison.
My Sime boiler keeps showing AL 28 after I reset it — what does that mean?
If AL 28 returns every time you reset the boiler, it means there is a persistent hardware problem the boiler cannot recover from on its own. Repeated resets will not resolve an electronic fault and may cause further stress to the board. Switch the boiler off at the isolator and arrange for a Gas Safe engineer to inspect it as soon as possible, particularly during cold weather when a loss of heating is a health concern.
Is it worth repairing a Sime boiler with an AL 28 fault, or should I replace it?
This depends largely on the age of the boiler. If it is under ten years old and otherwise in good condition, a PCB replacement is a reasonable investment that should restore reliable operation. For boilers over 15 years old, other components are likely reaching the end of their service life too, and the money spent on a PCB repair could go towards a more efficient modern replacement. Ask your engineer for an honest assessment of the boiler's overall condition before committing to a repair.