Sime AL 96 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 96 fault code mean?
AL 96 is a blocking (permanent) lockout fault indicating that the boiler's main electronic control board (PCB) has detected an internal problem it cannot recover from automatically. Depending on the specific Sime model, this can mean one of three things: an internal board component protection fault (the PCB has found an issue within its own circuitry and shut itself down to prevent further damage), an unstable combustion feedback signal error (the board is receiving inconsistent data from components involved in the combustion process), or a communication fault between the PCB and a connected bus device such as a Sime remote control or zone controller. Because this is a permanent lockout, the boiler will not restart on its own — a manual reset is required after the underlying cause has been resolved. The display may show 'RESET' alongside the fault code to prompt this.
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 damaged PCB (internal component failure) Common
The control board itself has developed a fault — this could be burnt-out components, oxidised contacts, or a failed onboard chip. This is particularly common after a period of repeated lockouts, as each hard shutdown creates minor electrical stress on the board over time.
- Water ingress on the PCB Common
A leak from the pump, pressure relief valve, or primary heat exchanger can drip onto the control board below. Even a small amount of moisture on PCB circuitry can cause short circuits, corrosion, and permanent component failure.
- Wiring connection fault Common
Loose, corroded, or damaged wiring between the PCB and key components — particularly the fan-to-board cable — can disrupt communication and trigger this error. The board interprets a missing or erratic signal as an internal fault.
- Power surge or voltage instability Sometimes
A spike on the mains supply can overwhelm and damage PCB components. UK homes with poor earthing or old consumer units are more vulnerable. Even a brief surge during a thunderstorm can be enough to corrupt the board.
- Communication fault with Sime bus remote control Sometimes
On Sime models fitted with an OpenTherm or proprietary bus remote control or zone controller, a disconnected, faulty, or incorrectly wired bus cable can cause the boiler to log a PCB communication error. The two-wire non-polarised bus line is sensitive to breaks or poor connections.
- PCB configuration or memory fault after replacement Rare
If the PCB has recently been replaced or the boiler has experienced a significant power event, the board's configuration parameters may have been lost or corrupted. On Sime boilers this typically requires the engineer to reset the board to defaults (PAR49 set to 1) and then re-enter the required parameters.
How to fix it
- Attempt a manual reset — once or twice only DIY safe
AL 96 is a permanent lockout, so the boiler will not restart on its own. Press and hold the reset button (your boiler display may show 'RESET' as a prompt) to attempt a restart. Do this no more than twice. If the fault returns immediately or within a short time, further resets will not help and could mask a worsening problem. Move on to the next steps.
- Check that the gas supply to your home is working DIY safe
Confirm that other gas appliances in your home (hob, gas fire) are working normally. If there is no gas supply, contact your gas supplier. If gas is available elsewhere but the boiler still shows AL 96, the fault is electronic rather than gas-supply related.
- Check and top up system pressure if low DIY safe
While not directly linked to AL 96, low system pressure can cause the PCB to log secondary errors. Check the pressure gauge — it should read between 1 and 1.5 bar when the system is cold. If it is below 1 bar, top up using the filling loop and bleed any air from radiators as needed. Re-check after topping up.
- Inspect visually for obvious leaks near the boiler DIY safe
Look around the base of the boiler casing and underneath for any signs of dripping water. Do not remove the boiler casing yourself. If you can see active dripping or wet patches, turn off the boiler and contact a Gas Safe engineer promptly, as water near the PCB is a serious concern.
- Check the remote control or room thermostat connection DIY safe
If your Sime boiler is fitted with a Sime bus remote control or external controller, check that all cables between the controller and the boiler appear intact and undamaged at the wall connection point. You can try temporarily disconnecting the remote control unit (do not open the boiler casing) to see if the fault clears — consult your remote control's user guide for how to do this safely.
- Gas Safe engineer: inspect all internal wiring and cable connections Gas Safe engineer
The engineer will open the boiler casing and systematically check each wiring loom — particularly the fan-to-PCB cable, sensor wiring, and bus communication cables. Loose plugs or corroded terminals are straightforward fixes that may resolve the fault without a board replacement.
- Gas Safe engineer: visual and electrical inspection of the PCB Gas Safe engineer
The board should be examined closely for burnt contacts, discolouration, swollen capacitors, and signs of moisture damage. The engineer will also use a multimeter to verify the board is receiving a stable mains supply and that output voltages are within specification.
- Gas Safe engineer: reset or reconfigure the PCB parameters Gas Safe engineer
If the fault is a configuration or memory issue (common after a power event or board replacement), the engineer can restore the board to factory defaults — on many Sime models this involves setting parameter PAR49 to 1, after which PAR1 and PAR2 will need to be correctly configured for the installation. This must only be done by a qualified engineer familiar with Sime commissioning procedures.
- Gas Safe engineer: replace the PCB if found to be faulty Gas Safe engineer
If inspection and testing confirm the control board is damaged beyond recovery, replacement is the only option. Sime PCBs are model-specific — the engineer will source the correct board and, once fitted, commission it to the correct parameters before carrying out a full functional test.
- If the fault persists, contact Sime Technical Assistance directly Gas Safe engineer
For AL 96 faults classified specifically as internal board protection errors, Sime's own installation manuals direct engineers to contact the Sime Technical Assistance Centre. Your engineer should do this if the fault cannot be resolved through standard diagnostics and component replacement.
Parts you may need
- Sime main PCB (model-specific) · from £290
- Boiler wiring loom / cable harness · from £55
- Sime bus remote control unit · from £80
- Pump (if leak onto PCB is the root cause) · from £95
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 £300–£600, depending on the underlying cause.
Frequently asked questions
Can I fix Sime AL 96 myself by resetting the boiler?
You can attempt a reset — and it is worth trying once or twice, as a temporary power glitch can occasionally trigger a PCB communication error that clears on reset. However, AL 96 is a permanent blocking lockout, which means the boiler has detected a genuine internal fault. If the code returns after one or two resets, further attempts will not help. At that point you need a Gas Safe registered engineer to diagnose the underlying cause, whether that is a wiring fault, water damage, or a failed control board.
How much does it cost to fix a Sime AL 96 fault in the UK?
For most homeowners, the total cost — including call-out, labour, and parts — falls between £300 and £600. If the fault turns out to be a loose or corroded wiring connection, you may pay only the call-out and labour charge (typically £60–£100 plus an hour or two of time). If the PCB itself needs replacing, the board alone can cost £180–£400 depending on the specific Sime model, plus labour. On older or less common models where parts are harder to source, costs can edge higher, but for most current Sime models the repair sits comfortably within that £300–£600 range.
Is it worth repairing a Sime boiler showing AL 96, or should I replace it?
This largely comes down to the age of the boiler and its service history. If your Sime is under 10 years old, has been serviced annually, and the repair estimate is well under half the cost of a replacement boiler, repair is almost always the better option. If the boiler is over 12–15 years old, has a history of repeated faults, or the PCB replacement quote is approaching £600 or more, it is worth getting a new boiler quote alongside the repair estimate — modern A-rated condensing boilers are significantly more efficient and come with manufacturer warranties of 5–10 years.
Could a remote control or room thermostat be causing Sime AL 96?
Yes, on Sime models that use a bus-connected remote control or zone controller (communicating via OpenTherm or Sime's own two-wire bus protocol), a fault on that communication line can cause the PCB to log a communication error, which may appear as AL 96. If the fault appeared shortly after a remote control was installed, disconnected, or damaged, mention this to your engineer — they will check the bus wiring as part of their diagnostic. In some cases, simply correcting the remote control wiring resolves the fault without any board replacement.