Sime AL 18 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 18 fault code mean?
AL 18 appears on Sime boilers when the PCB has detected a problem with the central heating flow NTC (temperature) sensor. The display shows 'AL' followed by '18' and the light bar turns red, indicating a protective lockout. The boiler cannot safely monitor the flow temperature — typically because the sensor has developed an open-circuit or short-circuit condition — so it shuts down to prevent overheating or damage. NTC sensors work by changing their electrical resistance as temperature rises; if the PCB receives a signal that falls outside expected parameters, it raises the alarm. A Gas Safe registered engineer will need to test and most likely replace the sensor, or in rarer cases investigate the wiring harness or PCB.
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 sensor Common
Over time the NTC thermistor itself deteriorates and can fail completely, producing an open-circuit or short-circuit reading that the PCB flags as AL 18. This is by far the most common cause and is usually a straightforward, low-cost fix.
- Loose, corroded, or damaged wiring to the sensor Sometimes
The cable and connector linking the flow NTC sensor to the PCB can suffer from vibration, corrosion, or heat damage. A poor connection produces an erratic or out-of-range resistance reading identical to a failed sensor, so a competent engineer will always check the wiring before condemning the sensor itself.
- PCB fault causing misreading of the sensor signal Rare
If a new sensor and confirmed good wiring still result in AL 18, the PCB may be at fault — either a damaged input circuit or a broader board failure. This is the least common root cause and the most expensive to resolve.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button (usually marked with a flame or reset symbol) for a few seconds, then release and allow up to 60 seconds for the boiler to attempt a restart. If it locks out again displaying AL 18, do not keep resetting — repeated resets will not fix an underlying sensor fault and may mask the problem.
- Check your system pressure DIY safe
Look at the pressure gauge on the front of the boiler. Sime recommend a normal cold-fill pressure of around 1–1.2 bar (shown in the blue zone). If pressure is below 0.8 bar, top up via the filling loop following the guidance in your user manual. Low pressure alone is unlikely to cause AL 18, but it is worth ruling out before calling an engineer.
- Check your gas supply is active DIY safe
Confirm other gas appliances in your home (hob, gas fire) are working normally. If there is a wider gas supply issue, contact your gas supplier rather than a heating engineer.
- Inspect the sensor wiring and connectors Gas Safe engineer
A Gas Safe engineer will visually check the wiring loom running from the CH flow NTC sensor to the PCB, looking for loose terminals, signs of corrosion, heat damage, or chafing. Any damaged wiring will be repaired or replaced at this stage.
- Test the NTC sensor resistance with a multimeter Gas Safe engineer
The engineer will disconnect the sensor and measure its resistance across the range of expected temperatures. As water temperature rises, the resistance of an NTC thermistor falls in a predictable curve. A reading that is open-circuit, short-circuit, or significantly off the manufacturer's specification confirms the sensor must be replaced.
- Replace the CH flow NTC sensor Gas Safe engineer
If testing confirms a faulty sensor, the engineer will fit a new OEM or compatible NTC sensor. This is a relatively inexpensive part (typically £5–£30) and the labour involved is modest — the most common outcome for AL 18.
- Inspect and test the PCB if the fault persists Gas Safe engineer
Should a new sensor and verified wiring still not clear the fault, the engineer will assess the PCB for damage or failure. A PCB replacement is significantly more costly, so a thorough engineer will exhaust sensor and wiring checks first.
- Commission and test the boiler Gas Safe engineer
After the repair, the engineer will follow the Sime reset procedure (pressing reset once the alarm is displayed), then run the boiler through a full heating and hot water cycle to confirm AL 18 does not return and that flow temperatures are being read correctly.
Parts you may need
- CH Flow NTC Temperature Sensor (Sime compatible) · from £18
- Sensor wiring harness / connector · from £25
- PCB (Printed Circuit Board) for Sime Murelle / Format range · from £280
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 £110–£280, depending on the underlying cause.
Frequently asked questions
Can I fix the Sime AL 18 fault myself?
The safe DIY actions are limited to a single reset attempt and checking your system pressure is in the correct range (1–1.2 bar). The actual diagnosis and repair — testing the NTC sensor, checking wiring, and replacing parts — must be carried out by a Gas Safe registered engineer. Working on boiler components yourself is both dangerous and a legal requirement to avoid in the UK.
How much will it cost to fix AL 18 on my Sime boiler?
In most cases AL 18 is caused by a faulty CH flow NTC sensor, which is an inexpensive part (around £5–£30). Including a Gas Safe engineer's callout and labour, most people pay between £110 and £280 for a sensor replacement. If the wiring harness also needs attention the cost sits towards the top of that range. In the rarer situation where a PCB replacement is needed, costs can rise to £350–£500 or more — but this outcome is uncommon for AL 18 specifically.
Will my Sime boiler come back on after resetting AL 18?
It may restart briefly after a reset, but if the underlying NTC sensor or wiring fault has not been fixed it will lock out again — often within minutes of firing up. The Sime manual advises that after any repair the reset procedure must be followed to clear the alarm. Avoid resetting the boiler more than two or three times without a proper diagnosis, as this can put extra strain on other components.
How urgent is AL 18 — can I leave the boiler off for a few days?
AL 18 is a lockout code, meaning the boiler is not running until the fault is cleared. This is not a gas-leak emergency, but it does mean you have no central heating or hot water. During colder months, leaving an unheated property for several days risks frozen pipes, so it is advisable to get an engineer booked promptly. In the short term, isolate the boiler using its isolator switch and use electric or portable heating as a temporary measure.