Sime AL 14 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 14 fault code mean?
When your Sime boiler displays AL 14, it has detected a problem with the flue or exhaust gas temperature sensor (sometimes called the NTC smoke probe). Depending on the specific model, this can mean the sensor is reading an open-circuit condition, a short-circuit condition, or that the flue gas temperature has exceeded a critical threshold (typically above 140°C). In all cases, the boiler locks out permanently as a safety measure and will not restart until the fault is cleared and a manual reset is performed. This code appears across several Sime ranges including the Format, Brava, Planet, and ALU HE — always cross-reference your model's installation manual to confirm the exact definition, as AL codes can vary between product lines.
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
- Faulty or degraded NTC flue/exhaust sensor Common
The NTC smoke probe monitors the temperature of the gases leaving the boiler. Over time these sensors degrade, drift out of calibration, or fail outright, causing the boiler's control board to receive a reading that is out of range or absent entirely. This is the most frequent root cause of the AL 14 lockout.
- Loose or damaged wiring to the flue sensor Common
Even if the sensor itself is fine, a loose connector, corroded terminal, or chafed wire in the harness running to the flue NTC probe can produce the same open-circuit or erratic signal that triggers AL 14. Vibration and heat cycling over years of use make connector problems surprisingly common.
- Blocked or obstructed flue terminal Sometimes
If the exhaust flue terminal outside your property is blocked — by a bird nest, debris, ice, or heavy cobwebbing — combustion gases cannot escape properly. This causes flue gas temperatures to climb abnormally, tripping the sensor's upper-limit protection and triggering a lockout. You can carry out a visual inspection of the external terminal yourself.
- Poor system water circulation Sometimes
Insufficient flow through the heat exchanger means heat is not being carried away efficiently, which can push exhaust temperatures above the safe limit even when the sensor and flue are otherwise intact. A failing pump, stuck zone valve, or severely sludged system can all restrict circulation enough to cause this.
- PCB fault misreading sensor signal Rare
In a small number of cases the sensor and wiring are both healthy, but the boiler's printed circuit board is incorrectly interpreting the signal — for example due to a damaged input circuit. This is the least common cause but worth considering if sensor and wiring checks come back clear.
How to fix it
- Check the external flue terminal for obvious blockages DIY safe
Go outside and locate where the flue exits the building — usually a white plastic terminal on an external wall or through the roof. Look for bird nests, a build-up of debris, heavy cobwebbing, or any physical damage that could be restricting the flow of exhaust gases. Do not attempt to dismantle the flue; a simple visual check is all that is needed here. If you spot a blockage you cannot safely clear from the outside without touching boiler components, note it and report it to your engineer.
- Reset the boiler (no more than two to three times) DIY safe
Once you have completed your visual checks, follow your Sime boiler's reset procedure — this is usually a dedicated reset button held for a few seconds, or powering the unit off at the socket, waiting a couple of minutes, and powering it back on. If the boiler fires up and runs normally, monitor it over the next few hours. If AL 14 returns immediately or after a short run, do not keep resetting — repeated resets without fixing the underlying cause can mask a genuine safety issue.
- Check system pressure is within the normal operating range DIY safe
Look at the pressure gauge on your boiler. It should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top it up using the filling loop following the instructions in your boiler's user guide. Low pressure on its own will not cause AL 14, but it can contribute to poor circulation which may play a role. Do not overfill beyond 2 bar.
- Inspect and test the NTC flue sensor and its wiring harness Gas Safe engineer
A Gas Safe engineer will locate the flue NTC probe, check its electrical resistance with a multimeter, and compare the reading against the manufacturer's specification for the current flue temperature. They will also inspect the full wiring harness for loose connectors, corrosion, or damage. If the sensor is out of specification it will be replaced; if wiring is the culprit, connectors will be cleaned or the harness repaired.
- Inspect the flue internally and confirm it is clear Gas Safe engineer
Your engineer will carry out a more thorough check of the flue run — not just the terminal — to confirm there is no internal blockage, condensate trap issue, or damage that could be causing gases to back up and overheat. This is especially important on installations where the terminal is not easily visible from ground level.
- Assess system circulation and water flow Gas Safe engineer
If the flue and sensor checks do not identify the fault, the engineer will evaluate whether poor water circulation is causing abnormally high flue gas temperatures. This may involve checking pump operation, testing zone valves, and assessing whether a system flush is required to clear sludge restricting flow through the heat exchanger.
- Call a Gas Safe registered engineer Gas Safe engineer
If the boiler does not clear the lockout after your own checks and a couple of resets, contact a Gas Safe registered engineer to carry out a full diagnosis. AL 14 involves components directly linked to combustion safety and gas operation — repairs must be carried out by a qualified professional. You can verify an engineer's registration at gassaferegister.co.uk.
Parts you may need
- Sime NTC flue/exhaust gas temperature sensor (smoke probe) · from £35
- Sensor wiring harness connector set · from £15
- Sime boiler PCB (control board) · 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 £100–£250, depending on the underlying cause.
Frequently asked questions
Can I reset my Sime boiler myself when AL 14 appears?
Yes, you can attempt a reset — turn the boiler off, wait a couple of minutes, and restart it. If the fault clears and the boiler runs normally, keep an eye on it over the next day or two. However, if AL 14 comes back straight away or returns within a short time, stop resetting and call a Gas Safe engineer. Repeatedly resetting without fixing the underlying sensor or flue issue will not resolve the problem and could mask a genuine safety concern.
How much does it typically cost to fix a Sime AL 14 fault in the UK?
For the most common scenario — a Gas Safe engineer visiting, diagnosing an NTC flue sensor fault, and replacing the sensor — you can expect to pay roughly £100 to £250 including parts and labour. The sensor part itself is usually £15–£50, with the rest being call-out and labour. If the investigation reveals a wider issue such as a PCB fault, costs can rise significantly — a replacement PCB can be £200–£350 for the part alone plus fitting. Emergency or out-of-hours call-outs will also push the total higher.
Why does the AL 14 code vary between different Sime boiler models?
Sime uses the 'AL' prefix across several boiler ranges, but the numbering is not always identical from one product line to the next. AL 14 is most consistently associated with the flue NTC temperature sensor across the Format, Brava, and Planet ranges, but on some models the same AL number may describe a slightly different condition. This is why it is always worth checking your specific model's installation manual or contacting Sime UK directly if you want an exact definition for your particular boiler.
Could a blocked flue really cause AL 14 on a Sime boiler?
Yes. The flue NTC sensor monitors exhaust gas temperature as a safety check. If the flue terminal or flue run is partially blocked — by a bird nest, debris, or ice in freezing weather — combustion gases cannot escape freely and their temperature rises. When the reading exceeds around 140°C the boiler's safety logic triggers a permanent lockout and displays AL 14. Checking the external flue terminal for obstructions is one of the few sensible DIY checks you can carry out before calling an engineer.