Sime AL 43 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 43 fault code mean?
The AL 43 fault code on a Sime boiler indicates a problem with the central heating return sensor, known as the SR sensor. This is an NTC (Negative Temperature Coefficient) thermistor clipped onto the return pipework that feeds cool water back into the boiler from your heating circuit. The boiler's PCB continuously reads the resistance from this sensor — as water temperature rises, the sensor's resistance drops — and uses that data to modulate the burner and keep the system operating safely. When the PCB detects a reading that is out of range, implausible, or entirely absent (open circuit), it triggers AL 43 and locks the boiler out as a precaution. This code appears on several Sime ranges including the Brava One ErP, Murelle PRO HE MkII, and Brava SLIM BFR series. It sits within a family of related sensor fault codes — AL 40, AL 41, AL 42, and AL 44 — each covering a different sensor on the same boiler platform.
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 return sensor Common
The SR thermistor itself has reached the end of its service life or has developed an internal fault, causing it to produce a resistance reading that falls outside the boiler's acceptable range. This is the most frequent root cause of AL 43 and typically requires straightforward sensor replacement by an engineer.
- Loose, corroded, or damaged wiring connection Common
The electrical connector joining the SR sensor to the boiler's wiring harness may have worked loose, suffered corrosion at the terminals, or sustained physical damage. Even a marginally poor connection can cause intermittent or permanently incorrect readings at the PCB, triggering the lockout.
- Corrosion or sludge on the sensor contact surface Sometimes
The NTC sensor clips directly onto the outside of the return pipe, so it relies on good metal-to-metal contact to get an accurate temperature reading. If the pipe surface is corroded or coated in sludge, the sensor effectively reads a false temperature. A system that has never been power-flushed is more susceptible to this.
- PCB fault misreading the sensor signal Rare
If the sensor and all its wiring test within specification but AL 43 persists, the fault may lie with the PCB itself incorrectly interpreting the sensor's signal. PCB issues are less common but should be investigated once all other causes have been ruled out by an engineer.
How to fix it
- Reset the boiler once or twice DIY safe
On Sime boilers, when the display shows the alarm code alongside a RESET prompt, press the reset button to attempt a restart. If the fault clears and does not reappear within a short period, the cause may have been a brief connection glitch. Limit resets to two or three attempts — repeated resets without identifying the underlying problem will not fix it and may mask a genuine issue.
- Check that your gas supply is live DIY safe
Make sure other gas appliances in the property (such as a hob or fire) are working normally. If there is no gas at other appliances, contact your gas supplier rather than resetting the boiler further.
- Have an engineer inspect the SR sensor wiring and connector Gas Safe engineer
A Gas Safe registered engineer will trace the wiring from the return sensor back to the PCB, checking for loose terminals, corroded pins, and any visible damage to the cable insulation. Loose connectors are sometimes all that is causing the fault and can be resolved without replacing any components.
- Have the NTC return sensor tested and replaced if necessary Gas Safe engineer
Using a resistance meter, the engineer will compare the sensor's actual readings against the resistance-temperature table specified in the Sime service documentation. If the sensor is out of tolerance or open circuit, it will be replaced. The SR sensor is a low-cost part and this is the most common resolution for AL 43.
- Ask the engineer to inspect and clean the sensor contact area Gas Safe engineer
If there is evidence of sludge or corrosion on the return pipework where the sensor sits, the engineer will clean the contact surface to restore accurate thermal transfer. They may also advise a power flush if the system water is heavily contaminated, as ongoing debris can cause repeat sensor faults.
- Have the PCB checked if replacing the sensor does not resolve the fault Gas Safe engineer
Should the boiler continue to display AL 43 after a confirmed good sensor has been fitted and all wiring checks out, the engineer will test the PCB's ability to correctly process the sensor signal. PCB diagnosis and potential replacement is specialist work requiring proper test equipment.
- Call a Gas Safe registered engineer Gas Safe engineer
All of the diagnostic and repair work required to resolve AL 43 must be carried out by a Gas Safe registered engineer. This includes sensor replacement, wiring repairs, and any PCB work. Do not attempt to access internal boiler components yourself.
Parts you may need
- Sime SR NTC return temperature sensor (thermistor) · from £20
- Wiring harness connector / sensor plug · from £12
- Sime PCB (main circuit 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 fix Sime AL 43 myself?
The reset button is the only step a homeowner should attempt. Everything else — testing the sensor's resistance, replacing the NTC thermistor, checking wiring continuity, or diagnosing the PCB — must be done by a Gas Safe registered engineer. Accessing internal boiler components without the correct qualifications is unsafe and also invalidates any remaining warranty.
How much does it cost to fix a Sime AL 43 fault in the UK?
For the most common fix — replacing the faulty SR return sensor — you can typically expect to pay between £100 and £250 once labour and the part are combined. The sensor itself is inexpensive (usually under £30), so the majority of the bill is the engineer's time. If the PCB turns out to be faulty, the part alone can cost £250–£350 or more, so the overall bill would be higher in that scenario. Prices in London and the South East tend to be at the upper end of these ranges.
What is the difference between AL 43 and the other AL 40–44 codes on a Sime boiler?
These codes all relate to temperature sensor faults but each targets a different sensor in the system. AL 40 and AL 41 typically cover the central heating flow sensor (SM), AL 42 and AL 43 relate to the central heating return sensor (SR), and AL 44 is associated with the flue or exhaust fumes sensor (SF). Because the codes are closely related, an engineer will often check all sensor connections at the same service visit.
Will my Sime boiler restart on its own after an AL 43 lockout?
No. AL 43 is a hard lockout — the boiler will stay off until it is manually reset. You can press the reset button once or twice to see if the fault was a temporary glitch, but if the AL 43 code returns after resetting, the underlying sensor or wiring issue needs to be diagnosed and repaired by a Gas Safe engineer before the boiler will run reliably again.