Sime AL 02 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 02 fault code mean?
The AL 02 fault code on a Sime boiler indicates that the water pressure in the central heating circuit has dropped below the acceptable minimum threshold. When the boiler detects this, it locks out as a safety measure and displays AL 02 — often alongside a flashing red light — to prevent damage to internal components such as the pump and heat exchanger. The boiler will not operate again until the pressure issue is resolved and the boiler is reset. This code is commonly seen on models such as the Sime Format DGT HE combi, sometimes persisting even after an initial top-up attempt if there is an underlying fault such as a failing pressure switch or expansion vessel.
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
- Genuinely low system pressure Common
The most frequent trigger. Pressure can drop naturally over time through small weeps at joints or radiator valves, after radiators have been bled, or following a discharge from the pressure relief valve. The system gauge will typically read below 1.0 bar when this is the cause.
- Faulty water pressure switch Sometimes
If you have topped the system up to the correct pressure (1.0–1.5 bar) but the AL 02 code persists and the boiler will not reset, the pressure switch itself may have failed. It is either stuck in the open position or has developed an internal fault, so it incorrectly reports low pressure to the PCB even though the actual pressure is fine.
- Failing or undercharged expansion vessel Sometimes
The expansion vessel absorbs pressure fluctuations as the water in the system heats and cools. If its internal diaphragm has perforated or it has lost its nitrogen charge, the vessel can no longer do this job properly. The result is that pressure swings wildly — dropping sharply when the boiler is cold — and the AL 02 code keeps returning shortly after topping up.
- System leak Sometimes
A pinhole leak on a radiator, a weeping joint, or a corroded section of pipework can cause the system to lose pressure repeatedly. If you find yourself needing to top up more than once a month, a leak is the likely culprit and needs tracing professionally before the fault will stay resolved.
- Air trapped in the system Rare
Significant air pockets can give an inaccurate pressure reading and affect circulation. Bleeding radiators removes trapped air, which can in some cases allow the pressure to stabilise at the correct level.
How to fix it
- Check the pressure gauge reading DIY safe
Look at the pressure gauge on the front of the boiler (or on the pipework beneath it). A reading below 1.0 bar confirms low pressure is the immediate problem. If the gauge already shows 1.0–1.5 bar, the issue may be a faulty pressure switch rather than genuinely low pressure — skip to step 4.
- Top up the system pressure using the filling loop DIY safe
Locate the filling loop — usually a short braided silver hose with one or two valves, found beneath the boiler. Make sure the boiler is switched off and has cooled down. Slowly open the filling loop valve(s) a quarter-turn at a time and watch the pressure gauge rise. Stop when the needle reaches 1.2–1.5 bar. Close both valves firmly. Never exceed 2.0 bar. If your boiler has a keyless filling link rather than a traditional loop, follow the same principle using the built-in lever.
- Bleed any radiators that feel cold at the top DIY safe
Trapped air can contribute to pressure instability and poor circulation. Use a radiator bleed key to release air from any radiators that feel cooler at the top than the bottom. Have a cloth handy. After bleeding, re-check the system pressure and top up again if it has dropped below 1.0 bar.
- Reset the boiler DIY safe
Once pressure is between 1.0 and 1.5 bar, reset the boiler using the reset button (refer to your Sime manual for the exact location — on most Sime models it is a dedicated button or held-down mode switch). Do not attempt to reset the boiler more than two or three times. If it locks out again quickly, stop and call an engineer.
- Monitor pressure over 24–48 hours DIY safe
After a successful reset, keep an eye on the pressure gauge over the next day or two. If pressure holds steady above 1.0 bar, the top-up has resolved the issue. If it drops again, there is likely a leak, a failed expansion vessel, or another underlying fault that requires a professional to investigate.
- Call a Gas Safe registered engineer Gas Safe engineer
If the AL 02 code returns after topping up, if you cannot locate or operate the filling loop safely, if pressure drops repeatedly, or if the boiler will not reset despite correct pressure, contact a Gas Safe registered engineer. They will pressure-test the system for leaks, inspect and recharge or replace the expansion vessel if needed, test and replace the water pressure switch if faulty, and carry out any necessary repairs to seals, joints, or pipework. Do not attempt to access or repair any internal boiler components yourself.
Parts you may need
- Water pressure switch · from £35
- Expansion vessel (typical combi size) · from £80
- Expansion vessel diaphragm/bladder · from £25
- Filling loop assembly · from £20
- Pressure relief valve (PRV) · from £15
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–£350, depending on the underlying cause.
Frequently asked questions
I've topped up the pressure but the AL 02 code keeps coming back — why?
If the pressure drops again within a few days or weeks, there are two likely explanations. The first is a leak somewhere in the system — even a slow weep at a joint or radiator valve will gradually drain pressure. The second is a failing expansion vessel: if its internal diaphragm has split, the vessel can no longer cushion pressure changes, so the system loses pressure every time the boiler cools down. A Gas Safe engineer can pressure-test the system to find leaks and check the expansion vessel charge with a simple gauge. Do not keep topping up repeatedly without getting to the root cause, as adding fresh water repeatedly introduces oxygen that accelerates internal corrosion.
How much does it cost to fix a Sime AL 02 fault?
For most households, the repair falls between £100 and £350. A straightforward top-up and reset done by an engineer on a call-out typically costs £80–£150. Replacing a faulty water pressure switch usually comes to £100–£250 including parts and labour. Expansion vessel replacement is the most common mid-range repair, averaging £150–£350. Repairing a minor system leak generally sits in the £150–£250 range. Very complex leak repairs involving re-piping, or awkward boiler access, can cost more — but these are the exception rather than the rule.
Is it safe to keep resetting the boiler when AL 02 appears?
You can try resetting the boiler two or three times once you have topped the pressure up to 1.2–1.5 bar. If it locks out again promptly each time, stop resetting it. Repeatedly forcing a boiler to run when it has a genuine pressure fault risks damaging the pump, heat exchanger, or other internal components. At that point the fault needs professional diagnosis.
Can I fix the AL 02 fault myself, or do I need an engineer?
Topping up the system pressure via the filling loop and resetting the boiler are safe DIY tasks that homeowners can carry out themselves. If these steps clear the fault and pressure remains stable, no engineer visit is needed. However, if the fault persists, pressure keeps dropping, or any internal component such as the pressure switch or expansion vessel needs attention, a Gas Safe registered engineer must do the work. It is illegal for an unregistered person to carry out gas appliance repairs in the UK.