Sime AL 03 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 03 fault code mean?
AL 03 indicates that the water pressure inside your sealed heating system has climbed too high. When the boiler detects this condition it locks out — shutting itself down safely — and shows 'AL 03' on the display alongside a red light bar. Normal operating pressure for most UK sealed systems sits between 1.0 and 1.5 bar when cold; AL 03 typically triggers once pressure climbs well above that range, often at or above 2.5–3 bar. The lockout is a protective measure: sustained high pressure can damage seals, valves and other internal components.
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
- System overfilled via the filling loop Common
The most frequent trigger. If the filling loop was opened too far during a top-up — or was not fully closed afterwards — excess water is pushed into the sealed circuit, sending pressure above the safe threshold. Even a loop that is only slightly weeping can gradually raise pressure over hours or days.
- Failed expansion vessel Common
The expansion vessel uses an air cushion separated by a rubber diaphragm to absorb the natural pressure rise as water heats up. If the diaphragm splits or the vessel loses its pre-charge, there is nowhere for expanding water to go, so pressure spikes each time the boiler fires. You may notice the pressure rising noticeably when the heating is on and water being discharged from the pressure relief valve outlet pipe.
- Split plate heat exchanger Sometimes
On some Sime combi models the plate heat exchanger can develop a crack, allowing mains cold water (which is at mains pressure) to cross over into the low-pressure heating circuit. The tell-tale sign is pressure that keeps rising even after you have bled the system and disconnected the filling loop — the water has to be coming from somewhere else.
- Filling loop valve not fully isolating Sometimes
A deteriorating or incorrectly installed filling loop valve can allow a small but continuous trickle of mains water into the system even when you believe it to be closed. Pressure climbs slowly and the fault recurs after each reset.
- Faulty pressure sensor Rare
The sensor that reports system pressure to the boiler's control board can develop a fault and send an erroneously high reading, triggering AL 03 even when actual pressure is within normal limits. An engineer can confirm this by comparing the sensor reading against a reliable mechanical gauge.
How to fix it
- Check the pressure gauge reading DIY safe
Look at the built-in pressure gauge or display on your Sime boiler. If the reading is clearly above 2.5 bar, the system is overpressurised. If the gauge reads within the normal 1.0–1.5 bar range when cold, the fault may be a sensor issue rather than genuinely high pressure — note this down for the engineer.
- Confirm the filling loop is fully closed DIY safe
Locate the filling loop — usually a braided or chrome link between two valves beneath the boiler. Make sure both isolation handles are firmly closed (perpendicular to the pipe). If the loop is the type that screws on, remove and store it away from the boiler to prevent accidental use.
- Bleed a radiator to release excess pressure DIY safe
If pressure is genuinely high, carefully bleeding a radiator will release some water and bring pressure back towards the 1.0–1.5 bar range. Use a radiator bleed key on the bleed valve (usually at the top of the radiator). Open it slowly — water, not just air, may come out. Close it once pressure on the gauge has dropped to around 1.2–1.5 bar. Do not bleed more than one or two radiators without rechecking the gauge, and never drop pressure below 0.8 bar.
- Reset the boiler DIY safe
Once pressure is back in the normal range, press and hold the reset button (or follow your model's reset procedure as described in your user guide) for around 3 seconds. The boiler should restart. If it fires up and runs normally, monitor the pressure gauge over the next few hours to see whether pressure climbs again.
- Watch for pressure rising again after the reset DIY safe
If pressure begins creeping back up within an hour or two — particularly as the heating runs and water warms — this strongly points to a failed expansion vessel or a cross-contamination fault such as a split plate heat exchanger. Do not continue resetting the boiler repeatedly; arrange an engineer visit.
- Do not attempt repairs to the expansion vessel, pressure sensor, plate heat exchanger or any internal components Gas Safe engineer
Re-charging or replacing the expansion vessel, diagnosing a faulty pressure sensor, replacing a split plate heat exchanger, and any work on gas or electrical components inside the boiler must be carried out by a Gas Safe registered engineer. Attempting these yourself is unsafe and will invalidate your boiler warranty.
- Call a Gas Safe registered engineer Gas Safe engineer
If the fault persists after the checks above, or if pressure rises again shortly after being corrected, book a visit from a Gas Safe registered engineer. They will pressure-test the expansion vessel, inspect the filling loop and plate heat exchanger, verify the pressure sensor reading, and carry out whichever repair is needed.
Parts you may need
- Expansion vessel (internal or external replacement) · from £45
- Expansion vessel diaphragm / bladder · from £20
- Water pressure sensor · from £22
- Filling loop assembly · from £18
- Plate heat exchanger (Sime combi models) · 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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I just keep bleeding radiators every time AL 03 comes back?
Bleeding radiators is a useful short-term step to bring pressure down, but it does not fix the underlying reason pressure keeps rising. If AL 03 returns repeatedly, something is continuously pushing water into the system — most likely a weeping filling loop valve or a failed expansion vessel. Repeatedly venting pressure without addressing the root cause can eventually leave your system undercharged and risks causing further problems. Book an engineer to investigate.
How much does it cost to fix AL 03 on a Sime boiler?
Most AL 03 repairs fall in the range of £120–£380 including labour. A straightforward pressure adjustment or expansion vessel re-charge sits towards the lower end, while an expansion vessel replacement typically costs £150–£280. A pressure sensor swap is usually £100–£180 all-in. A split plate heat exchanger is the expensive outlier — replacement parts and labour can reach £400–£600 or more, at which point it is worth getting a new boiler quote for comparison.
What is a normal boiler pressure for a Sime boiler?
When the heating is cold, pressure should sit between 1.0 and 1.5 bar — 1.2 bar is a common target. As the system heats up, pressure naturally rises slightly, but should not exceed around 2.0–2.5 bar when fully warm. AL 03 triggers when pressure climbs beyond the boiler's safe threshold, typically above 3 bar. Check your pressure gauge monthly and top up or bleed as needed to keep it in the normal range.
Why does my Sime boiler pressure keep rising by itself even after I release it?
If pressure climbs back up on its own — especially without you touching the filling loop — the most likely culprits are a failed expansion vessel diaphragm or a cracked plate heat exchanger leaking mains-pressure water into the heating circuit. Both require a Gas Safe engineer to diagnose and repair. A failed expansion vessel is more common; a split heat exchanger is less frequent but worth ruling out if the pressure rises unusually quickly and the expansion vessel checks out fine.