Sime AL 11 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 11 fault code mean?
The AL 11 code appears on Sime boilers — including the Brava One HE 25 and Brava Slim HE — when the PCB has detected a problem with the heating circuit (CH) flow temperature NTC thermistor. This small sensor continuously reports water temperature to the control board; if the PCB receives an implausible reading, detects an open or short circuit in the sensor circuit, or determines that the flow temperature has exceeded safe operating limits, it triggers AL 11 and locks the boiler out. The boiler will not restart automatically — a manual reset is required, but if the underlying fault is not resolved, it will lock out again. A Gas Safe registered engineer will need to inspect the sensor, its wiring, and the wider system to find the root cause.
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 NTC thermistor Common
The flow temperature sensor itself has developed an open or short circuit, often caused by age, water ingress into the sensor pocket, or a build-up of limescale or magnetite reducing thermal contact. The PCB reads a value outside the expected range and triggers the lockout.
- Loose, corroded or damaged wiring connector Common
The plug connecting the NTC thermistor to the wiring harness has worked loose, corroded over time, or been physically damaged. Even a slightly poor connection can produce an erratic resistance reading that the PCB interprets as a sensor fault.
- Overheating lockout (overheat condition) Sometimes
Rather than a sensor fault, the boiler genuinely overheated — meaning water temperature in the heat exchanger exceeded the safe threshold. This can happen if the circulation pump has failed or is running slowly, if sludge or scale is restricting water flow through the system, or if system pressure has dropped too low. The boiler locks out to protect itself.
- Short circuit or open circuit in the sensor cable harness Sometimes
The wiring between the NTC thermistor and the PCB has developed a fault — a chafed cable touching metalwork, a break in the conductor, or heat damage near the heat exchanger. This mimics a failed sensor even if the sensor itself is healthy.
- PCB input fault Rare
In a small number of cases the PCB's own NTC input circuit fails, causing it to misread a perfectly good sensor. This is typically only confirmed after the sensor and wiring have both been tested and found to be in good condition.
How to fix it
- Check that your gas supply is working DIY safe
Before assuming the boiler is the problem, confirm that other gas appliances in your home (hob, fire) are working normally. If you have no gas at all, contact your gas supplier rather than an engineer.
- Allow the boiler to cool, then attempt a single reset DIY safe
Press and hold the reset button on your Sime boiler as described in your user manual. Wait about 60 seconds after releasing it to see whether the boiler fires up. If it locks out again with AL 11 on the display, do not attempt further resets — repeated resets without fixing the root cause can mask a developing fault. Stop and call a Gas Safe engineer.
- Check and top up system pressure if it is below 1 bar DIY safe
Look at the pressure gauge on the boiler. If the needle is below 1 bar, the system is low on water — low pressure can restrict flow and contribute to overheating lockouts. Use the filling loop to top up to approximately 1–1.5 bar following the instructions in your user manual. This step will not fix a genuine sensor fault but rules out a straightforward pressure-related cause.
- A Gas Safe engineer will inspect and test the NTC thermistor Gas Safe engineer
The engineer will locate the flow temperature sensor on the heating circuit pipework close to the heat exchanger, remove the connector, and measure the sensor's resistance at a known ambient temperature, comparing the result against Sime's published resistance-temperature table for that sensor. An out-of-range reading confirms sensor failure and the part will be replaced.
- A Gas Safe engineer will check wiring continuity back to the PCB Gas Safe engineer
If the sensor tests within specification, the engineer will trace the cable harness from the sensor back to the PCB, checking for continuity, chafing, corrosion at the connector, and any signs of heat or moisture damage. A faulty section of harness will be repaired or replaced.
- A Gas Safe engineer will investigate any overheating root cause Gas Safe engineer
If AL 11 was triggered by a genuine overheat condition rather than a sensor circuit fault, the engineer will check that the circulation pump is running at the correct speed, inspect for sludge or scale restricting flow (particularly at the heat exchanger), check the expansion vessel charge pressure, and verify that all radiator and zone valves are functioning correctly.
- A Gas Safe engineer will assess the PCB if sensor and wiring are both healthy Gas Safe engineer
As a final step, if the NTC sensor and its wiring harness both check out correctly but the fault persists, the PCB's sensor input circuit will be investigated. PCB replacement is a more significant repair and the engineer may discuss whether it is cost-effective relative to the age of the boiler.
- Call a Gas Safe registered engineer if the fault persists after one reset Gas Safe engineer
If your boiler continues to display AL 11 after a single reset attempt, book a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Do not continue resetting the boiler repeatedly — this will not fix the underlying fault and could delay diagnosis.
Parts you may need
- NTC flow temperature thermistor (Sime compatible) · from £25
- Sensor wiring harness / connector · from £20
- Circulation pump (if overheating root cause) · from £85
- Expansion vessel (if contributing to overheat) · from £45
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–£320, depending on the underlying cause.
Frequently asked questions
Can I fix Sime AL 11 myself by just resetting the boiler?
A single reset is worth trying — occasionally a momentary voltage spike or minor glitch can trigger AL 11 and a reset clears it for good. However, if the boiler locks out again with the same code, the reset button will not cure the underlying fault. Continuing to reset it repeatedly risks masking a developing problem and may cause further damage. At that point you need a Gas Safe registered engineer to diagnose and repair the fault properly.
How much does it typically cost to fix a Sime AL 11 fault?
For an independent Gas Safe engineer, most AL 11 repairs come to somewhere between £150 and £320, covering call-out, one to two hours of labour, and the replacement NTC sensor (typically £20–£40 for the part itself). If the fault turns out to be caused by a failed circulation pump or a dirty heat exchanger requiring a power flush, costs will be higher. Sime's own fixed-price repair scheme starts at £350 including VAT and covers all labour and internal parts except the main heat exchanger — which can be worthwhile if you want a 90-day return guarantee.
What is an NTC thermistor and why does it cause a lockout?
NTC stands for Negative Temperature Coefficient — it is a type of resistor whose electrical resistance decreases as temperature rises. Your Sime boiler's PCB continuously measures this resistance to calculate the water temperature in the heating circuit. If the resistance reading falls outside the expected range (due to a failed sensor, a wiring fault, or an actual overtemperature condition), the PCB cannot safely determine what the water temperature is, so it shuts the boiler down and displays AL 11 to protect the heat exchanger and pipework from potential damage.
Could AL 11 mean my boiler overheated rather than a faulty sensor?
Yes — AL 11 can be triggered either by a sensor circuit fault or by the boiler genuinely reaching an unsafe temperature. If the overheat route is the cause, common culprits include a sluggish or failed circulation pump, heavy sludge or limescale build-up reducing water flow through the heat exchanger, or very low system pressure. A Gas Safe engineer will determine which scenario applies by testing the sensor resistance and checking flow rates and pump operation. If overheating is confirmed, fixing only the sensor will not prevent the code from returning.