Sime AL 08 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 08 fault code mean?
AL 08 indicates that your Sime boiler has detected a problem within the flame detection circuit — the electronic system responsible for confirming whether the burner has successfully lit and is producing a stable flame. When this circuit cannot reliably verify flame presence, the boiler locks out as a safety precaution and displays AL 08. This is distinct from AL 06 (no flame detected at all) and AL 07 (safety thermostat intervention): AL 08 points specifically to a fault in the detection and verification electronics or the components directly involved in that process, such as the ionisation electrode, its wiring, or the PCB logic that interprets the flame signal.
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
- Faulty or contaminated ionisation (flame sensor) electrode Common
The ionisation electrode sits in the burner flame and passes a small electrical current to confirm combustion. Over time it can become coated with oxidation or combustion deposits, or the electrode tip can wear and degrade, producing a weak or absent signal that the control board interprets as a circuit fault. This is by far the most frequently identified cause of AL 08.
- Damaged or deteriorated electrode wiring and connections Common
The low-voltage signal cable running from the ionisation electrode to the PCB is susceptible to heat damage, chafing, and loose connector pins. A break or poor connection anywhere along this cable will interrupt the flame signal and trigger AL 08. Engineers often find the connector at the board end has worked loose over time.
- Intermittent or insufficient gas supply Sometimes
If gas pressure to the boiler fluctuates or drops, the burner may light momentarily and then falter. The detection circuit sees a flame appear and then vanish in a pattern that does not match a clean ignition sequence, causing the boiler to lock out with a circuit-related fault code. Check that other gas appliances in the property are working normally.
- Blocked or partially obstructed burner Sometimes
Corrosion, debris, or a build-up of combustion residue on the burner can cause uneven or unstable combustion. This produces an erratic ionisation current that the PCB cannot interpret as a valid flame signal, even though the burner is technically alight.
- PCB fault Rare
If the flame detection circuitry on the printed circuit board itself has failed — for example due to a failed component or moisture ingress — it will be unable to process the electrode signal correctly even when the electrode and wiring are in good condition. PCB faults are usually only confirmed after other causes have been ruled out.
How to fix it
- Check the gas supply to your property DIY safe
Before assuming a boiler fault, confirm that gas is reaching your home. Try lighting a gas hob or another gas appliance. If there is no gas supply, contact your gas network provider (Cadent, SGN, Wales & West, or Northern Gas Networks depending on your region). Do not attempt to reset the boiler until normal gas supply is confirmed.
- Reset the boiler (up to two or three times) DIY safe
Once any obvious issue has been addressed, press and hold the reset button as described in your Sime boiler's user guide until the display clears. Allow the boiler to complete a full ignition sequence before judging whether the fault has cleared. If AL 08 returns after two or three resets, further resetting will not help and may mask a worsening fault — stop and call an engineer.
- Visually inspect around the boiler for obvious signs DIY safe
Look for any visible signs of water leaks near the boiler casing, signs of burning or scorching on accessible external wiring, or any obviously disconnected cables. Do not remove the boiler casing or touch internal components. Report any observations to your engineer.
- Gas Safe engineer: inspect and clean or replace the ionisation electrode Gas Safe engineer
A registered engineer will remove the burner assembly, inspect the ionisation electrode for wear or contamination, and either clean it carefully or fit a new one. This is the most common resolution for AL 08 and is typically carried out as part of a standard boiler service visit.
- Gas Safe engineer: inspect all flame detection circuit wiring and connectors Gas Safe engineer
The engineer will trace the signal cable from the electrode through to the PCB, checking for heat damage, cracking, chafing, and loose or corroded connector pins. Faulty wiring or a loose connector is a common secondary cause that is easily missed without a thorough inspection.
- Gas Safe engineer: inspect and test the burner Gas Safe engineer
If the electrode and wiring are found to be in good condition, the engineer will check the burner itself for blockage, corrosion, or uneven gas distribution that could be causing unstable combustion and an unreliable flame signal.
- Gas Safe engineer: diagnose and test the PCB Gas Safe engineer
If all upstream components check out, the engineer will test the PCB's flame detection logic. A faulty control board will need to be replaced with a genuine Sime-compatible unit. The engineer should verify the new board is correctly configured for your specific boiler model before firing up the appliance.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
AL 08 involves the internal electronics and gas-side components of the boiler. Beyond the basic checks above, all diagnostic and repair work must be carried out by a Gas Safe registered engineer. You can verify an engineer's registration at gassaferegister.co.uk. Do not use the boiler until the fault has been properly diagnosed and resolved.
Parts you may need
- Ionisation / flame detection electrode · from £25
- Electrode signal cable / ignition lead · from £30
- Burner assembly · from £80
- PCB (printed circuit board) · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I reset my Sime boiler myself when AL 08 appears?
Yes, you can attempt a reset — press the reset button as shown in your user guide and allow the boiler to try a full ignition sequence. If AL 08 clears and the boiler runs normally, monitor it over the next day or two. However, if the code returns after two or three resets, repeated resetting will not fix the underlying fault and you should call a Gas Safe registered engineer. Persistent lockouts suggest a component genuinely needs attention.
What is the difference between Sime fault codes AL 06, AL 07, and AL 08?
These three codes are related but distinct. AL 06 means the boiler attempted to light but could not detect a flame at all — often a gas supply or ignition problem. AL 07 indicates the safety thermostat has cut in, usually due to overheating or a circulation issue. AL 08 specifically means the flame detection circuit itself has a fault — so the boiler cannot reliably read or process the signal from the ionisation electrode, even if gas and ignition are otherwise working. AL 08 therefore tends to point towards the electrode, its wiring, or the PCB rather than the gas supply or heat management side of the boiler.
How much does it typically cost to repair a Sime AL 08 fault in the UK?
For the most common cause — cleaning or replacing the ionisation electrode, including the engineer's call-out and labour — expect to pay roughly £120 to £220. If wiring or ignition leads also need replacement the cost may reach £250 to £350. These figures include a typical call-out fee, parts, and labour with VAT. A PCB replacement, which is a less common outcome, costs considerably more — usually £350 to £500 or beyond depending on the specific board — but this is only needed if all other components have been eliminated. Getting two or three quotes from Gas Safe engineers is always worthwhile.
Is it worth repairing an older Sime boiler showing AL 08, or should I replace it?
For a Sime boiler that is fewer than ten years old and in otherwise reasonable condition, repairing an AL 08 fault is almost always cost-effective, especially if the fix is an electrode or wiring replacement. If your boiler is an older non-condensing model — such as a Sime Friendly, Super 90, or Super 102 — it may be 15 to 20 years old and well past its expected service life. In that case, recurring faults combined with lower efficiency mean a replacement condensing boiler could save you money in the long run. A Gas Safe engineer can give you an honest assessment of whether repair or replacement makes more sense for your specific boiler's age and condition.