Sime AL 13 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 13 fault code mean?
The AL 13 code on a Sime boiler indicates that the control board has detected a problem with the flue gas (exhaust) temperature sensor — commonly referred to as the smoke probe or SF sensor. This sensor monitors the temperature of gases leaving the boiler through the flue. When the board receives a signal that is open-circuit, short-circuit, or simply outside the expected range, it triggers the AL 13 alarm and shuts the boiler down in lockout to prevent any potential damage. The fix typically involves inspecting the sensor's wiring connections, checking the flue for obstructions, or replacing the sensor itself.
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
- Loose or corroded wiring connection to the SF sensor Common
The connector at the smoke probe or at the PCB end of the wiring loom can work loose over time, or the terminals can corrode — especially in the damp environment near the flue. This gives the control board an intermittent or absent signal and is usually the first thing a Gas Safe engineer will check.
- Failed flue gas temperature sensor (smoke probe) Common
NTC thermistor-type sensors degrade with age and heat cycling. When the sensor element fails, its resistance moves outside the range the control board expects, triggering AL 13. Replacement of the SF sensor is a straightforward and relatively low-cost repair.
- Blocked or partially obstructed flue Sometimes
A restricted flue — caused by debris, bird nesting material, or a disconnected section of flue pipe — can cause unusually high or fluctuating exhaust temperatures that confuse the sensor and trip the alarm. The engineer should verify the flue is clear and correctly sealed.
- Damaged wiring loom Sometimes
Chafing, burning, or rodent damage to the wiring between the sensor and the PCB can produce the same open-circuit or short-circuit signal that triggers AL 13, even if the sensor itself is in good condition.
- PCB (control board) fault Rare
If the sensor, its wiring, and the flue all check out correctly, the PCB may be misreading or failing to process the sensor signal. This is an uncommon but possible cause, and it is the most expensive to remedy.
How to fix it
- Try resetting the boiler DIY safe
Press and hold the boiler's reset button for approximately three seconds, as described in the Sime manual. The boiler may restart normally if the fault was a momentary glitch. Do not reset more than two or three times — repeated lockouts with no successful restart mean the underlying fault needs a proper diagnosis.
- Check that the gas supply to the property is active DIY safe
Make sure the gas meter isolation valve is open and that other gas appliances in the home (such as a hob) are working. If there is no gas supply, contact your gas supplier rather than a boiler engineer.
- Inspect the flue terminal for obvious external blockages DIY safe
If safe to do so, visually check the external flue outlet for any debris, leaves, or bird nesting material blocking the opening. Do not attempt to dismantle any flue pipework — leave that to the engineer.
- Have a Gas Safe engineer inspect the SF sensor wiring Gas Safe engineer
A registered engineer will trace the wiring loom from the flue gas temperature sensor back to the PCB, looking for loose terminals, corrosion, chafing, or heat damage. Reconnecting or replacing a damaged connector often resolves the fault at minimal cost.
- Have the engineer test and, if necessary, replace the smoke probe (SF sensor) Gas Safe engineer
The engineer will measure the sensor's resistance with a multimeter and compare it against the expected values in the Sime technical documentation. If the reading is out of range, the SF sensor will be replaced. This is a relatively low-cost repair — the sensor component itself is inexpensive, so most of the charge is the engineer's labour.
- Have the flue system fully inspected for integrity and blockages Gas Safe engineer
The engineer should verify that the combustion air inlet, flue outlet, and all connecting duct sections are undamaged, correctly joined, and airtight. A partially disconnected flue can cause erratic exhaust temperatures that trigger AL 13 without the sensor itself being faulty.
- Have the PCB assessed if all other components test correctly Gas Safe engineer
If the sensor, wiring, and flue are all in good order but the fault persists, the control board is the remaining suspect. PCB replacement is significantly more expensive than sensor work — see the FAQ below for typical costs — and on an older Sime boiler it may be worth getting a like-for-like replacement quote first.
- Call a Gas Safe registered engineer Gas Safe engineer
If you have not already done so, book a Gas Safe registered engineer to diagnose and repair the fault. You can verify an engineer's registration at gassaferegister.co.uk. All work on the gas system, flue, and internal boiler components is a legal requirement to be carried out by a registered professional in the UK.
Parts you may need
- Flue gas temperature sensor (SF / smoke probe) — Sime compatible · from £25
- NTC thermistor sensor wiring loom / connector · from £18
- Sime PCB (control board) — model-specific · 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 £100–£280, depending on the underlying cause.
Frequently asked questions
Is the Sime AL 13 fault code dangerous?
AL 13 itself is a protective lockout — the boiler has shut itself down because it cannot confirm the flue exhaust temperature is within safe limits. While the boiler is in lockout it is not operating, so there is no immediate danger. However, you should not attempt to bypass the lockout or force repeated resets without resolving the underlying fault. Call a Gas Safe registered engineer to investigate.
How much does it typically cost to fix a Sime AL 13 fault in the UK?
For the most common causes — a loose wiring connection or a failed smoke probe — you can expect to pay roughly £100 to £200 including the engineer's labour and parts. If the flue requires attention as well, costs may reach the £200–£280 range. A PCB replacement, which is an uncommon outcome for this fault code, can add significantly more — typically £300 to £500 including the part and labour — and in that scenario it is worth weighing up the repair cost against the age of the boiler.
Can I fix Sime AL 13 myself?
The only checks a homeowner can safely carry out are: attempting a boiler reset (no more than two or three times), confirming the gas supply is on, and checking the external flue terminal for obvious physical blockages. All internal boiler work — including inspecting the sensor, testing wiring, or replacing any components — must be carried out by a Gas Safe registered engineer. This is a legal requirement in the UK, not just a recommendation.
My Sime boiler keeps returning to AL 13 after resetting — what does that mean?
If the boiler locks out with AL 13 repeatedly after resetting, the fault is persistent and a reset alone will not fix it. The most likely causes are a genuinely failed SF sensor, a wiring fault, or a flue integrity problem. Each time the boiler tries to start, it detects the same out-of-range sensor signal and shuts down again. At this point, resetting further is unlikely to help and you should arrange for a Gas Safe engineer to carry out a proper diagnosis rather than continuing to reset.