Sime AL 37 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 37 fault code mean?
The AL 37 code appears when the boiler's control board detects that the incoming mains electrical supply voltage has dropped below the minimum level the boiler needs to function safely. Rather than risk damage or unreliable operation, the boiler shuts itself down and displays AL 37 to flag the problem. This is fundamentally an electrical supply issue rather than a gas or heating-circuit fault, and Sime's own guidance directs users to contact their Technical Assistance Centre when this code appears.
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
- Low or unstable mains voltage from the grid Common
The UK nominal mains voltage is 230V, and network regulations permit it to drop as low as 207V. If your local grid is experiencing a brownout or sustained low-voltage condition, the boiler's control board will detect the shortfall and lock out. This is outside the boiler itself and is ultimately your Distribution Network Operator's responsibility to resolve.
- Loose, corroded, or damaged wiring at the boiler Common
A deteriorated supply cable, corroded terminal block, or loose connection at the boiler's electrical intake can create a voltage drop even when the grid supply is perfectly healthy. Vibration over time or a poorly made original installation are common culprits. An engineer can test actual voltage at the boiler terminals to confirm this.
- Shared or overloaded circuit Sometimes
If the boiler shares its circuit with other high-demand appliances rather than having a dedicated spur, the combined load can pull the available voltage below what the boiler requires. Relocating the boiler to a dedicated, correctly rated circuit resolves this.
- Faulty PCB misreading the supply voltage Sometimes
A degraded printed circuit board can produce erroneous fault codes, including voltage-related ones, even when the actual supply is adequate. If an engineer confirms the mains voltage at the boiler terminals is within range and all wiring is sound, the PCB becomes the likely culprit.
- Damage from a power surge or spike Rare
A sudden voltage spike — from a nearby lightning strike or grid switching — can damage the control board's voltage-monitoring circuitry, causing it to report a low-voltage fault repeatedly even after the mains supply has returned to normal.
How to fix it
- Check your fuse box and boiler isolator switch DIY safe
Go to your consumer unit (fuse box) and confirm the circuit breaker or fuse protecting the boiler hasn't tripped or blown. Also check that the boiler's local isolator switch — usually a double-pole switch on the wall near the boiler — is fully on. If a breaker has tripped, reset it once. If it trips again immediately, stop and call an electrician before doing anything else.
- Check whether neighbours are affected DIY safe
Knock on a neighbour's door or check online to see whether others on your street are experiencing low voltage or power issues. If there is a wider outage or brownout, contact your Distribution Network Operator (DNO) — find yours via the Energy Networks Association website. The DNO is legally required to maintain supply within tolerance and will investigate for free.
- Attempt a boiler reset DIY safe
If the supply appears normal, press and hold the reset button on the boiler for the duration specified in your Sime manual (typically three to five seconds). Wait around 60 seconds for the boiler to restart. If it fires up and runs normally, monitor it over the next day or two. If it locks out again with AL 37, do not keep resetting — repeated lockouts indicate an underlying fault that needs professional diagnosis.
- Have a Gas Safe engineer test the voltage at the boiler terminals Gas Safe engineer
A registered engineer will use a multimeter to measure actual voltage at the boiler's electrical terminals. This quickly distinguishes a genuine supply problem from a wiring fault or a PCB issue. Do not attempt to open the boiler casing or test wiring yourself.
- Wiring inspection and repair by a qualified engineer Gas Safe engineer
If voltage at the terminals is low despite a healthy grid supply, the engineer will trace the supply cable back through the isolator and consumer unit to find loose connections, corroded terminals, or a undersized cable. Any remedial wiring work should be carried out by a suitably qualified person and, where required under Part P of the Building Regulations, notified to your local authority.
- PCB inspection and replacement if required Gas Safe engineer
Should mains voltage and all wiring check out correctly, the engineer will assess whether the control board itself is misreading or has been damaged. A faulty PCB must be replaced with a genuine Sime-compatible unit by an engineer — it is not a homeowner repair. Costs for this are higher than most other boiler repairs, so ask your engineer whether the age and overall condition of your boiler justify the spend.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If resetting the boiler and checking the consumer unit doesn't resolve AL 37, you need a Gas Safe registered engineer to carry out a proper electrical and component diagnosis. You can find registered engineers at the official Gas Safe Register website (gassaferegister.co.uk). Make sure to mention the AL 37 code when you call so the engineer can bring appropriate test equipment.
Parts you may need
- Sime PCB (control board) · from £220
- Boiler terminal block / connector strip · from £15
- Boiler supply cable and isolator switch · from £25
- Inline surge protector for boiler supply · from £30
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–£380, depending on the underlying cause.
Frequently asked questions
Is AL 37 a gas fault — do I need a Gas Safe engineer or an electrician?
AL 37 is an electrical fault, not a gas fault, so a Gas Safe registration is not strictly required to test mains voltage or repair supply wiring. However, because the work involves opening the boiler casing and testing its internal terminals, most boiler engineers handle this as part of a standard boiler service call. If the fault turns out to be a domestic wiring issue upstream of the boiler (for example a problem at the consumer unit), a Part P registered electrician should carry out that element of the work.
Can I just keep resetting the boiler to clear AL 37?
Resetting once or twice is fine as a first step — a brief grid dip can trigger the code and then resolve itself. But if the boiler locks out again within a short period, repeated resets will not fix the underlying problem and may mask a developing fault. Two or three resets without a lasting recovery is the point at which you should stop and call an engineer.
How much does it cost to fix Sime AL 37 in the UK?
For the majority of AL 37 cases the repair involves a wiring or connection fix, typically costing between £100 and £250 including the engineer's callout. If the fault is traced to a faulty PCB, costs rise considerably — expect £450 to £700 for the part and fitting, which is why it is worth getting a boiler replacement quote alongside any PCB repair quote if your boiler is ten or more years old. Engineers usually charge £60 to £100 for the initial diagnostic visit.
Could AL 37 be caused by a problem outside my home?
Yes. If your local Distribution Network Operator (DNO) is delivering voltage below the statutory minimum of 207V, your boiler will detect it and lock out regardless of the condition of your internal wiring or boiler components. If neighbours are also experiencing dim lighting or appliance issues, contact your DNO — they are responsible for maintaining supply voltage and will investigate free of charge. Your DNO can be identified via the Energy Networks Association postcode lookup tool online.