Sime AL 40 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 40 fault code mean?
AL 40 on a Sime boiler means the control board has detected that the frequency of the incoming mains electricity is wrong. UK mains runs at 50 Hz, and the board checks it because its timing, pump and fan control all assume a stable supply; if the measured frequency drifts outside tolerance the boiler stops and raises AL 40. Sime's documented advice is to contact the electricity network provider, because the fault is almost never inside the boiler: it points at the property's supply — a generator or inverter supply running off-frequency, a failing UPS or backup supply, or a genuine network disturbance. The companion code AL 37 covers low supply voltage; AL 40 is specifically frequency.
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
- Genuine supply disturbance Common
Network faults can push frequency briefly out of tolerance; the boiler protects itself and normally recovers when the supply stabilises. Repeated AL 40 with an otherwise normal supply warrants a call to your electricity network operator, per Sime's guidance.
- Boiler fed from a generator, inverter or UPS Sometimes
Off-grid or backup supplies that are poorly regulated commonly run off 50 Hz, and the boiler's check catches it. The fix is on the supply side, not the boiler.
- Board measurement fault Rare
Rarely the supply is fine and the board's own frequency measurement has failed — considered only when the supply has been verified healthy.
How to fix it
- Check whether other electronics show problems DIY safe
Flickering lights or other appliances misbehaving at the same time point to a real supply issue rather than the boiler.
- If the boiler is on a generator, inverter or UPS, say so DIY safe
Backup and off-grid supplies are the classic cause of frequency faults. The boiler needs a clean 230 V 50 Hz supply.
- Reset once with the OK/RESET button DIY safe
Hold the OK RESET button for more than 3 seconds. Sime boilers allow a limited number of consecutive resets (six) before locking out on AL 28, so if the code returns after one or two attempts, stop and book an engineer rather than using them all up.
- Have a Gas Safe registered engineer carry out Sime's documented remedy Gas Safe engineer
If AL 40 recurs on normal mains, follow Sime's advice: report it to your electricity network operator (the UK equivalent of the manual's 'network provider'), and have an engineer or electrician verify the supply at the boiler. Only if the supply proves healthy does attention turn to the control board.
Parts you may need
- Control PCB · from £200
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 £80–£250, depending on the underlying cause.
Frequently asked questions
What does AL 40 mean on a Sime boiler?
The board has measured the mains electricity frequency as outside tolerance (UK mains should be 50 Hz) and stopped as a precaution. Sime's manual directs this at the electricity supply rather than the boiler — it usually means a supply disturbance or a poorly regulated generator/inverter feed.
Why would my electricity frequency be wrong?
On grid mains it rarely is for long — brief network disturbances happen and the boiler recovers. Persistent AL 40 usually means the boiler is fed from a generator, inverter or UPS running off-frequency, or there is a genuine local network problem worth reporting to your distribution network operator.
Is AL 40 a boiler fault at all?
Usually not. Reset once and see if it recurs; if it does, investigate the supply before condemning any boiler part. Only when the supply is proven clean does the board's own measurement circuit come under suspicion.