Sime AL 07 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 07 fault code mean?
The AL 07 code appears on Sime boilers when the built-in safety thermostat (TS) has triggered a protective shutdown. This thermostat acts as a last line of defence against overheating — when it detects that temperatures inside the boiler have exceeded safe limits, or that a related component has failed, it cuts operation and locks the boiler out. On Sime displays, the fault is shown as "AL" followed by "07", accompanied by a red light bar. The boiler will not restart until the underlying cause has been identified and the fault is cleared. Because overheating can cause permanent damage to the heat exchanger, this code should always be investigated promptly rather than repeatedly reset.
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
- Blocked or seized pump impeller Common
The circulating pump moves heated water around the system. If the impeller becomes clogged with sludge or debris, or seizes after a period of inactivity, water flow drops sharply. Without adequate circulation, heat builds up rapidly in the heat exchanger and the safety thermostat trips. This is one of the most frequently found causes of AL 07 on Sime boilers.
- Air trapped in the heating system Common
Pockets of air in the pipework or heat exchanger interrupt the flow of water. The boiler continues to fire but cannot transfer heat away efficiently, causing localised overheating and safety thermostat intervention. Bleeding the radiators and checking system pressure can sometimes resolve this, though a Gas Safe engineer should confirm the root cause.
- Faulty or disconnected safety thermostat Sometimes
The safety thermostat sensor itself may have developed a loose wiring connection or an internal fault. When this happens it can send a false overtemperature signal to the PCB, triggering an AL 07 lockout even when actual boiler temperatures are normal. A Gas Safe engineer will check the connections and test or replace the component.
- Scale or sludge build-up on the heat exchanger Sometimes
In hard-water areas, limescale accumulates inside the heat exchanger over time, reducing its ability to transfer heat into the water. The trapped heat causes genuine overheating. Sludge from corroding pipework has a similar effect. A powerflush followed by inhibitor treatment is typically required alongside any component repair.
- Faulty NTC thermistor giving incorrect temperature readings Rare
The NTC thermistor monitors water temperature and reports to the PCB. If it drifts out of calibration or fails outright, the PCB may misread temperatures and instruct the safety thermostat circuit incorrectly. This can mimic a genuine overheat event and result in an AL 07 lockout.
How to fix it
- Check and restore system pressure if necessary DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop to top it up slowly to around 1.2 bar. Low pressure alone can impair circulation and contribute to overheating faults.
- Bleed the radiators to remove trapped air DIY safe
Using a radiator bleed key, open the bleed valve on each radiator starting with the one furthest from the boiler. Hold a cloth underneath and release air until water begins to flow steadily, then close the valve. Re-check boiler pressure afterwards and top up again if needed, as bleeding can lower it.
- Reset the boiler (no more than two or three times) DIY safe
Once you have topped up the pressure and bled the radiators, attempt a boiler reset using the reset button or procedure described in your user manual. If the AL 07 code clears and the boiler runs normally, monitor it over the next 24 hours. If the fault returns, do not keep resetting — repeated resets on an overheating fault can cause heat exchanger damage.
- Have a Gas Safe engineer inspect the circulating pump Gas Safe engineer
A qualified engineer will check whether the pump impeller is blocked, sticking, or has seized. They may be able to free a seized impeller or, if the pump has failed, replace it. This is not a homeowner task as it involves draining part of the system and working on internal components.
- Have a Gas Safe engineer test the safety thermostat and its wiring Gas Safe engineer
The engineer will check the thermostat sensor connections for corrosion or looseness and test the component electrically. A faulty or incorrectly reading safety thermostat will be replaced. Wiring faults will be repaired at the same time.
- Have a Gas Safe engineer check the NTC thermistor Gas Safe engineer
The engineer will use diagnostic equipment to verify that the NTC thermistor is reading temperatures accurately. A drifting or failed thermistor is relatively straightforward to replace and often resolves AL 07 faults that return after initial checks.
- If scale or sludge is suspected, arrange a powerflush Gas Safe engineer
A Gas Safe engineer or heating specialist can carry out a powerflush to clear sludge and scale from the system. Inhibitor should be added afterwards to protect against future corrosion. This work does not require Gas Safe registration itself but should be arranged as part of the overall repair to prevent the fault recurring.
- Call a Gas Safe registered engineer if the fault persists or recurs Gas Safe engineer
If the AL 07 fault continues after the above checks, or if you are not confident performing even the homeowner steps, contact a Gas Safe registered engineer. Overheating faults that are left unresolved risk permanent heat exchanger damage, which is a significantly more expensive repair. You can find your nearest Gas Safe engineer at gassaferegister.co.uk.
Parts you may need
- Sime safety thermostat (TS) · from £35
- Circulating pump (compatible replacement) · from £85
- NTC thermistor temperature sensor · from £20
- Magnetic system filter (e.g. Fernox TF1 or Adey MagnaClean) · from £75
- Central heating inhibitor (e.g. Fernox F1, 1 litre) · from £15
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–£380, depending on the underlying cause.
Frequently asked questions
Is it safe to keep resetting my Sime boiler when AL 07 appears?
No — you should attempt a reset once or twice to see whether the fault clears after straightforward checks such as topping up pressure or bleeding radiators. However, if AL 07 returns, stop resetting. This code indicates a safety thermostat intervention, and repeated resets on a boiler that is genuinely overheating can cause lasting damage to the heat exchanger, turning what might have been a £150 repair into one costing £500 or more.
Could a frozen condensate pipe cause the AL 07 fault on a Sime boiler?
Unlikely. A frozen condensate pipe typically produces its own specific fault code related to ignition or flue problems rather than an overtemperature lockout. AL 07 is specifically linked to the safety thermostat circuit. That said, if you are also seeing condensate-related symptoms in very cold weather, you can carefully thaw the external condensate pipe with warm (not boiling) water, then reset. If AL 07 persists, the thermostat circuit itself needs professional attention.
How much does it typically cost to fix a Sime AL 07 fault?
For the most common causes — pump issues, sensor faults, or trapped air — most homeowners pay between £120 and £380 including parts and labour. A safety thermostat or NTC thermistor replacement is towards the lower end of that range. A pump replacement sits in the middle. If scale or sludge is the underlying cause, add the cost of a powerflush (typically £300–£500 as a standalone job). In rare cases where prolonged overheating has damaged the heat exchanger, costs can reach £600–£900 or more — another reason not to ignore this fault code or keep resetting without investigation.
How do I find a Gas Safe engineer to fix my Sime boiler?
You can search the official register at gassaferegister.co.uk by postcode. Always ask to see the engineer's Gas Safe ID card before work begins — it shows which gas appliances they are qualified to work on. For Sime boiler repairs, look for engineers experienced with European boiler brands. Some Sime approved service agents can also be located through Sime's UK distributor.