Sime AL 15 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 15 fault code mean?
AL 15 appears on Sime boilers — including the Brava Slim HE and Brava One HE series — when the control board detects a problem with the domestic hot water (DHW) NTC temperature sensor. The PCB has identified that the sensor circuit is either open-circuit (no signal reaching the board, typically due to a broken wire or failed sensor element) or short-circuit (the sensor's internal resistance has collapsed, sending an invalid reading). Because the boiler can no longer reliably monitor hot water temperatures, it locks out and displays AL 15 to prevent unsafe operation. The red light bar will illuminate alongside the AL 15 code on the display.
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 DHW NTC sensor Common
Over time, NTC thermistors degrade through scale build-up, magnetite deposits, or water ingress into the sensor body. This alters the resistance characteristics the PCB expects, eventually causing it to read the sensor as open- or short-circuit and trigger AL 15. This is the most frequent cause of this fault code.
- Loose, corroded, or damaged wiring Common
The wiring loom connecting the DHW sensor to the PCB can develop loose plugs, corroded terminals, or chafed cable insulation — particularly in older boilers. Even a partially disconnected plug can produce an open-circuit reading indistinguishable from a failed sensor, causing a false AL 15 lockout.
- Short circuit within the sensor Sometimes
Internal breakdown of the sensor element causes its resistance to fall to near zero, which the PCB interprets as a short-circuit fault. The boiler locks out immediately because a shorted sensor cannot provide meaningful temperature data.
- PCB input circuit fault Rare
In rarer cases the sensor and its wiring are both sound, but the PCB's own sensor input channel has failed. The board misreads the signal and reports AL 15 even though the thermistor is working correctly. This is usually only confirmed after a new sensor fails to clear the fault.
How to fix it
- Check whether the fault clears with a boiler reset DIY safe
Press and hold the reset button on the boiler fascia as described in your Sime manual — typically for around 3 seconds until the lockout clears. If AL 15 returns promptly, the fault is persistent and a reset alone will not resolve it. Do not reset the boiler more than two or three times, as repeated resets on a genuine sensor fault will not fix the underlying problem.
- Confirm your hot water and heating demand is active DIY safe
Make sure the boiler's time and temperature controls are calling for hot water. A boiler that simply is not being asked to heat water will not display AL 15 — if the code is on the screen, the fault is genuine and not a settings issue.
- Book a Gas Safe registered engineer to inspect the DHW sensor wiring Gas Safe engineer
An engineer will remove the boiler casing and trace the wiring loom from the DHW NTC sensor back to the PCB, checking for loose plugs, corroded pins, or damaged insulation. Secure connections are re-made and any corroded terminals are cleaned or replaced before further diagnosis.
- Engineer to test DHW NTC sensor resistance with a multimeter Gas Safe engineer
With the sensor wires disconnected at the PCB end to eliminate stray resistance, the engineer measures the sensor's resistance in ohms and cross-references the reading against Sime's temperature-resistance specification table. A reading that is out of range — or open/short — confirms the sensor has failed and needs replacing.
- Engineer to replace the DHW NTC temperature sensor Gas Safe engineer
If the sensor is confirmed faulty, the engineer will fit a new genuine Sime DHW NTC thermistor. After fitting, the boiler is fired and the sensor readings are verified on the PCB before the job is signed off. Using a manufacturer-approved replacement ensures the resistance curve matches what the Sime PCB expects.
- Engineer to inspect the PCB if AL 15 persists after sensor replacement Gas Safe engineer
If a new sensor and confirmed-good wiring still fail to clear AL 15, the fault lies with the PCB's sensor input circuit. The engineer will assess whether the board can be repaired or needs replacing. Given the cost of a PCB, the age and overall condition of the boiler should be taken into account at this point.
- Call a Gas Safe registered engineer if you have not already done so DIY safe
All internal boiler work on a Sime appliance must be carried out by a Gas Safe registered engineer. You can verify any engineer's registration free of charge at gassaferegister.co.uk before booking. Do not attempt to open the boiler casing or disturb any internal components yourself.
Parts you may need
- Sime DHW NTC temperature sensor (thermistor) · from £35
- Sensor wiring harness / loom · from £30
- Sime PCB (control board) · 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–£220, depending on the underlying cause.
Frequently asked questions
Can I reset my Sime boiler to clear AL 15 myself?
Yes, a reset is always worth trying first — press and hold the reset button as described in your Sime manual. If AL 15 disappears and does not return, a transient electrical glitch may have caused it. However, if the code comes back within a short time, the sensor or its wiring has genuinely failed and resetting the boiler repeatedly will not fix it. In that case you need a Gas Safe engineer to diagnose and repair the fault.
How much does it cost to fix a Sime AL 15 fault in the UK?
For most homeowners, an AL 15 repair — covering the call-out, labour, and a replacement DHW NTC sensor — comes to roughly £100–£220. The sensor itself is one of the cheaper boiler components at around £15–£50, so labour makes up the bulk of the bill. If the PCB turns out to be the root cause rather than the sensor, the cost will be higher — a Sime PCB typically costs £180–£280 for the part alone, plus labour. Engineers in London and the South East generally charge 20–35% more than the national average.
Is it safe to use my boiler with AL 15 showing?
No. When AL 15 is active, the boiler has locked itself out and will not produce domestic hot water until the fault is cleared. This is a deliberate safety measure — without reliable temperature data from the DHW sensor, the boiler cannot safely regulate water temperature. Do not attempt to bypass the lockout; arrange for a Gas Safe engineer to diagnose and fix the fault before using the boiler normally again.
My Sime boiler is quite old — is it worth repairing an AL 15 fault?
If the fix is simply a new DHW NTC sensor, the repair is relatively inexpensive and almost always worth doing regardless of boiler age. However, if your Sime boiler is 15 years old or more and AL 15 turns out to be caused by a faulty PCB, it is worth asking your engineer for an honest assessment of the boiler's overall condition. Older non-condensing Sime models in particular can be less energy-efficient and more prone to further faults, so a replacement boiler may offer better long-term value than an expensive PCB repair.