Sime AL 10 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 10 fault code mean?
When your Sime boiler displays AL 10, it has detected a problem with the central heating flow NTC (Negative Temperature Coefficient) thermistor — the small temperature-sensing component mounted on the CH flow pipework inside the boiler. This sensor continuously feeds water-temperature readings to the PCB so the boiler can regulate heat output accurately, typically targeting between 60–80°C for a central heating circuit. If the PCB receives a reading that falls outside expected parameters — whether due to an open circuit, a short circuit, or a value that simply doesn't make sense — it classifies this as a sensor fault, displays 'AL' followed by '10' on the screen, turns the light bar red, and locks the boiler out as a safety precaution. The boiler will not fire until the underlying fault is resolved and the unit is 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
- Failed or degraded CH flow NTC thermistor Common
The NTC thermistor itself has reached the end of its service life or suffered internal failure. These small components do degrade over time, particularly in systems with water-quality issues, and will eventually produce readings that the PCB rejects entirely. This is the single most frequent cause of AL 10.
- Loose, disconnected, or corroded wiring connector Common
The plug-in connector between the NTC sensor and the wiring harness can work loose through vibration or may corrode if there is any dampness around the sensor pocket. A poor connection mimics a failed sensor as far as the PCB is concerned, triggering the same lockout.
- Water ingress or leak around the sensor pocket Sometimes
Even a slow weep from a nearby joint or the sensor gland itself can allow moisture to track along the wiring and cause corrosion at the connector pins. Check the area around the flow sensor for any signs of dried limescale or staining that could indicate a historic or ongoing drip.
- Scale or magnetite deposits affecting the sensor Sometimes
Heavy limescale in hard-water areas, or magnetite sludge in systems without adequate inhibitor, can insulate the sensor from the water it is supposed to measure, causing erratic or implausible temperature readings that trigger AL 10.
- PCB input circuit fault Rare
If a replacement NTC sensor and verified wiring still fail to clear the code, the PCB's own sensor-input circuit may have developed a fault. This is a less common cause but should be considered once all other possibilities have been ruled out by a Gas Safe engineer.
How to fix it
- Reset the boiler once DIY safe
Locate the reset button on your Sime boiler (consult your user manual if needed) and hold it for the duration specified — typically around three seconds. If the boiler fires up and runs normally, monitor it over the next hour. If AL 10 returns promptly, do not keep resetting; a persistent fault means the underlying cause needs professional attention.
- Check the boiler's system pressure DIY safe
Glance at the pressure gauge. A reading between 1.0 and 1.5 bar when the system is cold is normal. If pressure is low, top it up via the filling loop. A very low-pressure condition can sometimes cause secondary sensor-related faults, so it is worth ruling this out before calling an engineer.
- Inspect the wiring harness and connector at the CH flow NTC Gas Safe engineer
A Gas Safe engineer will remove the boiler casing and locate the NTC thermistor on the central heating flow pipe. They will check that the connector is firmly seated, look for any signs of corrosion or heat damage on the pins, and examine the cable run back to the PCB for chafing or breaks.
- Test the NTC sensor resistance Gas Safe engineer
Using a multimeter, the engineer will measure the resistance of the NTC thermistor at a known water temperature and compare the reading against Sime's specification table. An open circuit, short circuit, or reading that deviates significantly from the expected value confirms sensor failure and the need for replacement.
- Replace the CH flow NTC thermistor if required Gas Safe engineer
If testing confirms the sensor is faulty, the engineer will fit a replacement NTC thermistor. This often involves draining a small amount of water from the primary circuit. Once fitted, the connector and wiring route should be carefully re-secured to prevent the fault recurring.
- Evaluate the PCB if a new sensor does not resolve the fault Gas Safe engineer
Should AL 10 persist after a verified replacement sensor has been installed and the wiring checked, the engineer will assess the PCB's sensor-input circuitry. PCB replacement or repair is a more significant job; see the FAQ below for guidance on costs and whether repair versus boiler replacement makes more sense.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler has not cleared after your reset attempt, contact a Gas Safe registered engineer to carry out steps 2–5 above. You can verify any engineer's registration at gassaferegister.co.uk before they start work. Do not attempt to open the boiler casing or disconnect any components yourself.
Parts you may need
- CH flow NTC thermistor (Sime compatible) · from £18
- NTC sensor wiring harness/connector · from £25
- Sime PCB (model-dependent) · 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
Can I reset a Sime AL 10 fault myself?
Yes, you can attempt a single reset using the boiler's reset button. If AL 10 clears and the boiler runs normally, keep an eye on it. However, if the code comes back straight away or within a short time, the sensor or its wiring has a real fault that needs a Gas Safe registered engineer to diagnose. Repeatedly resetting without fixing the cause just delays the repair and can sometimes mask how serious the problem is.
How much does it cost to fix a Sime AL 10 fault in the UK?
For most households, the repair involves inspecting the wiring and replacing the CH flow NTC thermistor — a relatively straightforward job. Parts and labour together typically fall in the range of £100–£280. If the PCB also turns out to be faulty, that adds significantly more; PCB replacement on a Sime boiler generally costs £300–£500 including fitting, which is why it is worth exhausting all sensor and wiring checks first before replacing the board.
How long does an NTC sensor replacement take?
In most cases a Gas Safe engineer can replace a CH flow NTC thermistor within an hour, including draining a small amount of water from the circuit and testing the repair. It is a routine job rather than a major strip-down, so most engineers can complete it in a single visit.
Should I repair or replace my Sime boiler if it keeps showing AL 10?
If the boiler is less than around 10 years old and only the NTC sensor needs attention, repair is almost always the better value. If the boiler is older than 10–12 years, is showing multiple faults, and the PCB also needs replacing alongside the sensor, the combined repair cost can start to approach the cost of a new, more efficient boiler. In that situation it is worth getting a replacement quote alongside the repair quote and weighing up the long-term running costs.