Sime AL 06 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 06 fault code mean?
AL 06 on a Sime boiler signals that the boiler has attempted to ignite but failed to detect a flame — a condition known as an ignition lockout. The ionisation (flame-sensing) electrode, which continuously monitors whether combustion is taking place, has not confirmed a stable flame during the start-up sequence. As a safety measure the boiler shuts itself down and displays AL 06. Until the underlying cause is identified and resolved, the boiler will not restart automatically; you must press the Reset button once the fault has been addressed.
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
- Dirty or degraded ignition/ionisation electrode Common
Over time the electrode tip accumulates carbon deposits or corrodes, preventing it from either sparking reliably or detecting the presence of a flame. This is the single most frequent cause of AL 06 and is often picked up during an annual service.
- Gas supply interrupted or too low Common
If the gas meter isolation valve has been turned off, a prepayment meter has run out of credit, or there is a wider supply disruption in the area, the boiler cannot ignite at all. Other gas appliances in the home (hob, fire) will also fail to light.
- Frozen condensate pipe Sometimes
In cold weather the plastic condensate drain pipe running outside can freeze solid, causing the boiler to lock out. Although AL 06 is primarily an ignition fault, a frozen condensate blockage can trigger related shutdowns on some Sime models.
- Incorrect mains wiring polarity or poor earthing Sometimes
Sime boilers rely on mains earth to complete the ionisation detection circuit. If the live and neutral connections were swapped during installation, or if the earth connection is missing or corroded, the control board cannot sense the flame signal correctly and AL 06 will appear.
- Faulty ignition lead or HT cable Sometimes
The high-voltage lead carrying the spark from the transformer to the electrode can develop hairline cracks or loose terminals. A damaged lead causes intermittent or absent sparking without any obvious external sign.
- Defective ignition transformer Sometimes
The transformer steps mains voltage up to the level needed to produce a spark. If its output is weak or absent, ignition cannot occur. An engineer will check output voltage against Sime's specification.
- Gas valve fault Rare
A gas valve that sticks closed or opens insufficiently means no gas reaches the burner, so the electrode sparks into empty space. Gas valve faults are less common but result in persistent AL 06 that does not respond to electrode cleaning or reset.
- PCB (printed circuit board) failure Rare
The PCB orchestrates the entire ignition sequence and reads the ionisation signal. If it develops a fault it may misread or fail to send the correct signals, causing a false AL 06. PCB failure is relatively uncommon but represents the most costly outcome.
How to fix it
- Check that all gas appliances in your home have a working supply DIY safe
Try lighting a gas hob ring or check whether another gas appliance is working. If nothing lights, the problem is with your gas supply rather than the boiler. Contact your gas supplier or call the National Gas Emergency Service on 0800 111 999 if you smell gas.
- Check the boiler's system pressure DIY safe
Look at the pressure gauge on the boiler front. It should read between 1.0 and 1.5 bar when cold. If it is below 1 bar, top it up via the filling loop following the instructions in your Sime boiler manual. Low pressure alone rarely causes AL 06, but it is worth eliminating.
- Thaw a frozen condensate pipe if the weather is very cold DIY safe
If temperatures have recently dropped below freezing, locate the white plastic condensate pipe that exits the property (usually through an outside wall). Pour warm — not boiling — water over it, or wrap a hot-water bottle around it, until it flows freely. Never use a naked flame.
- Reset the boiler DIY safe
Once you have checked the points above, press and hold the Reset button (consult your Sime manual for the exact location on your model) until AL 06 clears and the boiler attempts a restart. Allow one full ignition cycle. If the lockout returns after one or two resets, stop — repeated failed attempts will not fix the underlying fault and could cause further issues.
- Have a Gas Safe engineer inspect and clean or replace the ignition and ionisation electrodes Gas Safe engineer
The engineer will remove the burner cover, inspect the electrode tip for carbon build-up or damage, clean it or fit a new genuine Sime electrode, and verify the gap is set to manufacturer specification. This is the most likely fix for AL 06 and is routinely carried out at annual service.
- Have the engineer check mains wiring polarity and earth continuity Gas Safe engineer
Using appropriate test equipment the engineer will confirm that live, neutral, and earth are correctly connected at the boiler terminal block. A reversed live/neutral or a missing earth will prevent the ionisation circuit from working even when all components are in perfect condition.
- Have the engineer test the ignition lead and HT transformer Gas Safe engineer
The high-tension lead will be inspected for cracking or loose terminals and the transformer output voltage measured. A faulty lead is straightforward and inexpensive to replace; a failed transformer will also need to be swapped out.
- Have the engineer test the gas valve operation Gas Safe engineer
If electrodes, wiring, and the transformer all check out, the engineer will verify that the gas valve is opening correctly and delivering adequate gas flow to the burner. A sticky or failed valve will require replacement.
- If all components test satisfactory, have the PCB assessed Gas Safe engineer
A Gas Safe engineer with Sime experience can carry out board-level checks or arrange a like-for-like replacement if the PCB is found to be misreading the ionisation signal. Given the cost of a PCB, a reputable engineer will exhaust all other possibilities first.
- Call a Gas Safe registered engineer if the fault persists after one or two resets Gas Safe engineer
You can find a Gas Safe registered engineer in your area at gassaferegister.co.uk. Ask whether they have experience with Sime boilers. Describe the AL 06 code when you call so the engineer can bring likely replacement parts.
Parts you may need
- Sime ignition/ionisation electrode · from £35
- HT ignition lead (high-tension cable) · from £25
- Ignition transformer · from £55
- Gas valve (Sime compatible) · from £120
- PCB (Sime model-specific) · from £180
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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I fix Sime AL 06 myself by just resetting the boiler?
You can try a reset — and it is worth doing once or twice after checking the gas supply and condensate pipe — but AL 06 is a safety lockout that will keep returning until the root cause is resolved. If the boiler relocks after one or two attempts, you need a Gas Safe registered engineer to diagnose and repair it. Repeatedly resetting without fixing the fault is not safe practice.
How much does it typically cost to repair a Sime AL 06 fault in the UK?
Most homeowners with AL 06 pay between £150 and £380 all-in (call-out, labour, parts, and VAT). Electrode cleaning or replacement at the lower end sits around £150–£220, wiring corrections around £100–£200, and an ignition transformer swap around £150–£300. A gas valve replacement typically costs £300–£450. In the less common situation where the PCB is at fault, costs can reach £400–£550 or more — worth weighing against the boiler's age and overall condition.
Why does my Sime boiler show AL 06 only in cold weather?
A persistent winter-only AL 06 strongly suggests a frozen condensate pipe. The condensate drain carries acidic water from the flue gases outside; when it freezes, the boiler can lock out. You can safely thaw the pipe yourself using warm water (see the steps above). If it keeps freezing each winter, an engineer can insulate the pipe or reroute the drain internally to prevent recurrence.
How do I prevent AL 06 coming back on my Sime boiler?
The best prevention is an annual service by a Gas Safe registered engineer who will clean the electrodes, check all electrical connections including earthing and polarity, verify gas pressure, and inspect the ignition components. Sime recommend yearly servicing for all their boilers, and it keeps the manufacturer's warranty valid. Also ensure the condensate pipe is insulated before winter if it runs in an exposed outdoor location.