Sime AL 01 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 01 fault code mean?
The AL 01 fault code on a Sime boiler signals that the water pressure within the central heating circuit has dropped below the minimum safe operating level. When the boiler detects this condition, it locks out — shutting itself down to prevent damage — and the display shows 'AL 01' alongside a red light bar. System pressure on a Sime boiler should normally sit between 1.0 and 1.5 bar when cold. Because AL 01 is a relatively broad code, the underlying cause can vary, and Sime's own documentation recommends qualified technical assistance to pinpoint the exact fault on the boiler in front of you.
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 system pressure — general pressure loss Common
The most straightforward cause: the water pressure in the sealed heating circuit has simply fallen, often gradually over time through minor weeping at joints or radiator valves. Check the pressure gauge on the boiler — if it reads below 1.0 bar, a simple top-up via the filling loop may clear the fault immediately.
- Faulty or waterlogged expansion vessel Common
The expansion vessel absorbs pressure changes as the system heats and cools. Over time the internal diaphragm can harden, split or lose its pre-charge of air, leaving the vessel waterlogged. This causes the system pressure to fluctuate wildly and drop repeatedly. Sometimes the vessel can be recharged; if the diaphragm has failed, full replacement is needed.
- Water leak in the heating circuit Common
A persistent or recurring pressure drop — particularly if you find yourself topping up more than once a month — strongly suggests a leak somewhere in the pipework, radiator valves, connections or at the boiler itself. Joints may weep slowly without leaving an obvious puddle, so a thorough inspection is required.
- Faulty automatic air vent Sometimes
The auto air vent (AAV) is designed to release trapped air from the system but can develop a weeping seal, allowing water to drip out slowly. If no obvious leak is found elsewhere, a defective AAV is a common culprit for gradual, unexplained pressure loss.
- Split plate heat exchanger Rare
On older Sime models, the plate heat exchanger can develop a crack or split, allowing mains cold water to bleed into the heating circuit. A tell-tale sign is pressure that appears to rise by itself even after the filling loop is disconnected — effectively the mains is feeding the circuit through the crack. This is a more serious and costly fault.
How to fix it
- Check the pressure gauge and top up via the filling loop DIY safe
With the boiler switched off and cool, locate the filling loop — typically a braided silver hose with one or two valves beneath the boiler. Open the valve(s) slowly, watching the pressure gauge, until the needle reaches between 1.0 and 1.5 bar. Close both valves firmly. Do not overshoot 2.0 bar. Switch the boiler back on and allow it to fire up normally.
- Reset the boiler DIY safe
Once pressure has been restored, the boiler may need a manual reset. Refer to your Sime manual for the reset button location — on most models it is held for a few seconds until the display clears. If the boiler restarts and runs normally, monitor the pressure gauge over the next few days to see if pressure holds steady.
- Monitor pressure over the following weeks DIY safe
If the pressure remains stable above 1.0 bar and AL 01 does not return, you may have resolved a one-off pressure drop. However, if you need to top up more than once a month, or if the pressure drops rapidly within days, this points to an underlying fault that needs professional attention.
- Inspect visible pipework and radiator connections for leaks DIY safe
Walk around the property and check accessible pipework, radiator valves and any visible joints for dampness, rust staining or mineral deposits, which can indicate a slow weep. Check under the boiler casing for signs of dripping. Do not open the boiler casing yourself.
- Arrange a Gas Safe registered engineer to investigate further Gas Safe engineer
If pressure keeps dropping, or if you cannot identify a visible leak, contact a Gas Safe registered engineer. They will pressure-test the system, inspect the expansion vessel pre-charge, check the auto air vent, and assess the heat exchanger for any internal splitting. All internal boiler work — including expansion vessel replacement, AAV replacement, and any pipework repairs within the boiler — must be carried out by a qualified professional.
Parts you may need
- Expansion vessel (heating circuit) · from £55
- Expansion vessel diaphragm / membrane · from £20
- Automatic air vent (AAV) · from £12
- Filling loop assembly · from £18
- Pressure relief valve (PRV / safety valve) · from £15
- Plate heat exchanger (Sime compatible) · from £120
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 £150–£450, depending on the underlying cause.
Frequently asked questions
Can I fix Sime AL 01 myself by topping up the pressure?
Topping up the pressure via the filling loop is a perfectly safe DIY task, and if a one-off pressure drop is the cause, this may resolve AL 01 straight away. However, if the pressure drops again within days or weeks, there is an underlying fault — a leak, a failed expansion vessel, or another component issue — that requires a Gas Safe registered engineer to diagnose and fix. Do not repeatedly top up and ignore the root cause, as running a leaking system can cause further damage.
How often should I need to top up my Sime boiler pressure?
On a healthy sealed heating system, you should rarely need to top up pressure at all — perhaps once a year at most, usually after bleeding radiators. If you are topping up more than once a month, or if the pressure drops rapidly, this is a sign of a leak or a failing expansion vessel and you should book an engineer promptly.
What does it cost to repair AL 01 on a Sime boiler in the UK?
For the most common causes, typical repair costs range from around £150 to £450 including parts and labour. Re-pressurising during a service call sits at the lower end. Expansion vessel recharging or replacement usually costs £180–£350. Leak tracing and repairs vary depending on where the leak is. A split plate heat exchanger is a more significant repair and can cost £400–£700 or more — if that is suspected on an older boiler, it is worth getting a replacement quote alongside a repair quote.
Why does my Sime boiler keep losing pressure even after I top it up?
Recurring pressure loss almost always points to either a water leak somewhere in the system, a failed expansion vessel diaphragm, or — less commonly — a cracked heat exchanger allowing cold mains water to enter and then escape. A Gas Safe engineer can pressure-test your system and inspect the expansion vessel pre-charge to identify which component is responsible. Ignoring repeated pressure loss risks further component damage and, in the case of a leak, potential water damage to your property.