Sime AL 09 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 09 fault code mean?
The AL 09 fault code (displayed as ALL 09 on some Sime models) means the boiler's flow switch has detected insufficient water circulation through the heating circuit. When circulation falls below the expected level, the boiler shuts down as a safety precaution to prevent the heat exchanger from overheating. If the condition persists for more than one minute, the boiler will pause for six minutes before attempting a restart. In plain terms, water is either not moving around your system at all, or not moving quickly enough for the boiler to consider it safe to fire.
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
- Faulty or seized circulation pump Common
The pump is responsible for pushing water around the heating circuit. If it has seized, lost power, or is running but not generating enough flow, the flow switch will not register adequate circulation and the AL 09 fault will appear. This is the single most common root cause of this code.
- Low system pressure Common
If the boiler pressure has dropped below around 1 bar, there may not be enough water in the system for the pump to move effectively. The pressure gauge on the boiler front panel will confirm whether this is the case.
- Airlocks in the heating circuit Common
Air trapped in radiators or pipework can disrupt water flow, preventing the pump from circulating water properly. Bleeding the radiators releases trapped air and restores normal flow.
- Blocked or dirty filter/strainer Sometimes
Many systems have a magnetic filter or strainer on the return pipe. If this becomes clogged with sludge, rust, or debris, it can restrict flow enough to trigger the fault.
- Closed isolation or zone valves Sometimes
If a service valve, zone valve, or isolator on the flow or return pipework has been accidentally left closed or is stuck shut, water cannot circulate freely. A visual check of all accessible valves is worthwhile.
- Debris or rust blocking the flow switch Sometimes
The flow switch itself can become fouled with system debris or corrosion, causing it to stick in either the open or closed position and report a false no-flow condition even when the pump is working correctly.
- Flow switch wiring fault Rare
Loose, corroded, or damaged wiring between the flow switch and the PCB can cause the control board to misread the flow switch signal, triggering the AL 09 code without there being a genuine circulation problem.
- Faulty PCB Rare
In rare cases where all other components have been tested and found to be functioning correctly, a defective printed circuit board may be misinterpreting signals or failing to command the pump to run at all.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If the needle is below 1 bar, the low water volume may be contributing to the circulation fault. Refer to your Sime manual and top up via the filling loop until the gauge reads around 1.2 bar, then attempt a reset.
- Reset the boiler (up to 2–3 times) DIY safe
Press and hold the reset button as described in your Sime manual. If the boiler fires and runs normally, monitor it over the next few hours. If it locks out again with AL 09, do not keep resetting — move on to further checks.
- Bleed your radiators to remove airlocks DIY safe
Turn off the heating and let the system cool. Using a radiator bleed key, open the bleed valve on each radiator (starting upstairs and working down) until water flows steadily with no air bubbles. Re-check the boiler pressure afterwards and top up if needed, as bleeding can cause a small pressure drop.
- Check all isolation and zone valves are fully open DIY safe
Inspect any accessible valves on the pipework near the boiler and around the system. A valve handle that is perpendicular to the pipe is closed; it should be in line with the pipe to be open. If any zone valves are motorised, confirm they are not stuck shut.
- Have a Gas Safe engineer inspect and test the circulation pump Gas Safe engineer
An engineer will check whether the pump is receiving power, running at the correct speed, and actually generating flow. A seized pump can sometimes be freed if caught early, but a failed pump will need replacing. Pump replacement on a Sime boiler typically costs £150–£350 including parts and labour.
- Have a Gas Safe engineer clean or replace the system filter Gas Safe engineer
A heavily silted magnetic filter can choke the system enough to cause the AL 09 fault. An engineer can clean or replace this as part of a service visit. If there is no filter fitted, they may recommend installing one to protect the boiler long-term.
- Have a Gas Safe engineer inspect and test the flow switch Gas Safe engineer
A qualified engineer can test the flow switch electrically without fully dismantling it. If it is found to be stuck due to debris, it can sometimes be cleaned; if it has failed, replacement parts are relatively inexpensive and the total repair including call-out and labour typically falls in the £80–£150 range.
- Have a Gas Safe engineer check the wiring and PCB Gas Safe engineer
If the pump, filter, and flow switch have all been checked and the fault persists, an engineer will inspect the wiring harness for damage or corrosion and test the PCB. PCB replacement is a significant cost (typically £200–£450 including labour) so an engineer may advise whether repair is economical given the age of the boiler.
- Call a Gas Safe registered engineer if the fault has not cleared Gas Safe engineer
If the AL 09 code returns after the homeowner checks above, or you are unsure about any step, contact a Gas Safe registered engineer. You can verify any engineer's registration at gassaferegister.co.uk. Do not remove the boiler casing or attempt internal repairs yourself.
Parts you may need
- Central heating circulation pump (compatible with Sime) · from £120
- Flow switch / differential pressure switch · from £35
- Magnetic system filter · from £60
- Boiler PCB (Sime) · from £180
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Sime AL 09 fault myself?
Some initial checks are safe for a homeowner: topping up the system pressure via the filling loop, bleeding radiators, and checking that any accessible valves are fully open. If those steps do not clear the fault, the likely causes — a faulty pump, blocked filter, or failed flow switch — all require a Gas Safe registered engineer to diagnose and repair safely. Do not remove the boiler casing or attempt internal work yourself.
What is the difference between AL 09 and ALL 09 on a Sime boiler?
They are the same fault. Some Sime display panels show the code as AL 09 while others, particularly older or larger models, display it as ALL 09 due to differences in the screen format. Both indicate that the flow switch has detected insufficient water circulation through the heating circuit.
How much does it cost to fix a Sime AL 09 fault in the UK?
The cost depends entirely on the root cause. Simple fixes like topping up pressure or bleeding radiators cost nothing. Cleaning or replacing a flow switch typically runs £80–£150 including a Gas Safe engineer's call-out and labour. Replacing the circulation pump is the most common paid repair and usually costs £150–£350 all-in. In rare cases where the PCB is at fault, replacement can cost £200–£450 or more — at which point it is worth considering the age of the boiler and whether a replacement is better value overall.
My Sime boiler keeps showing AL 09 after I reset it — what should I do?
Resetting the boiler two or three times is reasonable if you have just made a change such as topping up the pressure or bleeding a radiator. However, if the fault returns each time without any improvement, continued resetting will not solve the underlying problem and may cause further wear. Stop resetting and book a Gas Safe registered engineer to carry out a proper diagnosis. Persistent AL 09 faults most commonly point to a circulation pump that is on its way out, a blocked system filter, or a faulty flow switch.