Sime AL 05 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 05 fault code mean?
The AL 05 code on a Sime boiler indicates a problem with the delivery (flow) temperature sensor — the NTC thermistor that sits on the flow pipe coming out of the heat exchanger. This probe continuously monitors the temperature of water being pushed into the heating circuit, allowing the boiler's controls to manage burner output safely. When the boiler's PCB detects a reading from this sensor that is outside the expected range — or no reading at all — it triggers AL 05 and shuts the boiler down as a protective measure. The fault will remain displayed until the underlying problem is resolved and the boiler is manually 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 flow NTC thermistor Common
Over time, the NTC thermistor's resistance characteristics drift or the component fails outright. This causes the boiler's PCB to receive a signal that is either absent or wildly out of range, triggering the AL 05 lockout. This is by far the most frequent reason for this fault code appearing.
- Loose, corroded, or damaged wiring connections Common
The wiring harness and plug connecting the flow sensor to the PCB can work loose through vibration, or suffer from corrosion and moisture ingress — particularly around the sensor pocket on the flow pipe. A poor connection produces an intermittent or absent signal that the boiler interprets as a sensor fault.
- Water ingress into the sensor body Sometimes
If system water seeps into the sensor housing — for example from a weeping joint near the sensor pocket — it can cause a short or open circuit in the thermistor, producing an erratic or flat-line reading and triggering the alarm.
- Scale or sludge build-up around the sensor Sometimes
Heavy limescale or magnetite deposits in the flow pipework can insulate the sensor from the water temperature it is trying to measure, causing inaccurate readings. In hard-water areas or systems without inhibitor, this can gradually drive the sensor reading out of the acceptable range.
- PCB fault on the sensor input circuit Rare
If the sensor and all associated wiring test as electrically sound, a fault on the main printed circuit board — specifically in the analogue input circuit that reads the NTC signal — can produce a false AL 05. This is uncommon and is typically only diagnosed after a sensor replacement has failed to clear the code.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button until the boiler attempts to restart. A single reset is a reasonable first step — the fault may have been a transient signal glitch. If the AL 05 returns immediately or after the next heating cycle, the underlying problem needs investigation and further resets will not help.
- Check the boiler's system pressure DIY safe
Glance at the pressure gauge. Sime boilers typically require 1–1.5 bar when cold. If the pressure is below 0.5 bar the system may not circulate properly, which can stress temperature sensors. Top up via the filling loop if needed, then attempt one more reset. If pressure was normal, move on.
- A Gas Safe engineer inspects the sensor and wiring Gas Safe engineer
The engineer will locate the flow NTC thermistor on the outlet side of the heat exchanger, disconnect the wiring plug, and visually inspect both the sensor body and the wiring harness for signs of corrosion, burning, chafing, or water damage. Any damaged wiring is repaired or replaced at this stage.
- Electrical testing of the NTC thermistor Gas Safe engineer
Using a multimeter, the engineer measures the resistance across the sensor terminals at a known water temperature and compares the reading against Sime's resistance-temperature chart. A reading that is open circuit, short circuit, or significantly outside the expected range confirms a faulty thermistor requiring replacement.
- Flow NTC thermistor replacement Gas Safe engineer
The engineer will drain a small amount of water from the relevant section of the boiler, remove the old sensor, and fit a like-for-like replacement. Thermal contact compound should be applied correctly to ensure accurate heat transfer between the sensor and the pipe. The system is refilled, repressurised, and the boiler test-fired to confirm the fault has cleared.
- PCB investigation if fault persists after sensor replacement Gas Safe engineer
If AL 05 returns even after a confirmed good replacement sensor is installed, the fault lies upstream — in the PCB's sensor input circuitry. The engineer will test the signal voltage at the PCB terminals and, if the board is found to be faulty, advise on replacement or whether a boiler of this age is economically worth repairing.
- Contact a Gas Safe registered engineer Gas Safe engineer
If you have not already done so, arrange a visit from a Gas Safe registered engineer to diagnose and repair this fault. You can verify any engineer's registration at gassaferegister.co.uk. Do not continue attempting resets or operating the boiler with a lockout fault active.
Parts you may need
- Flow NTC thermistor (Sime compatible) · from £25
- Wiring harness / sensor plug · from £18
- Main PCB (control board) · from £180
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 a Sime AL 05 fault myself?
Yes, a single reset attempt is fine — press and hold the reset button and see if the boiler fires up and stays running. If AL 05 comes back within the same heating cycle or on the next attempt, stop resetting and call a Gas Safe engineer. Repeatedly resetting a lockout fault without fixing the root cause will not clear it and could mask a developing problem.
How much does it cost to fix a Sime AL 05 fault in the UK?
For a straightforward flow NTC sensor replacement, expect to pay roughly £100–£220 all in, covering parts and a Gas Safe engineer's labour. The sensor itself is inexpensive (typically £15–£40), so most of the cost is the call-out and time on-site. If the PCB turns out to be the culprit, a replacement board can push costs to £300–£450 or more — worth bearing in mind if your Sime boiler is already over 10–15 years old, as a new boiler may be a better investment.
Where is the flow (delivery) sensor located on a Sime boiler?
The flow NTC thermistor is fitted into a pocket or clip on the flow pipe immediately downstream of the heat exchanger — the pipe that carries heated water out towards the heating circuit. It is usually accessible once the boiler casing is removed. Its location means a minor leak or condensation around that area can sometimes cause water ingress into the sensor, so the engineer will check the surrounding pipework as well.
My Sime boiler keeps showing AL 05 after the sensor was replaced — what now?
If a new, correctly installed flow sensor does not clear the fault, the most likely remaining cause is a fault on the PCB itself — specifically the analogue input channel that reads the NTC signal. Less commonly, a wiring fault between the sensor and the PCB was missed during the first inspection. Ask your engineer to re-test the wiring continuity all the way back to the board and to check the reference voltage at the PCB sensor terminals before concluding the board needs replacement.