Sime AL 84 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 84 fault code mean?
AL 84 on a Sime boiler means the combustion controller has had to reduce the boiler's flow rate because it suspects low pressure on the mains gas supply. The self-regulating combustion system can feel when the gas side cannot sustain the demanded output — it compensates by derating, and it flags what it has done. Sime's documented response is to check the gas flow rate and to bleed air from the gas circuit. Genuine causes split between the supply (low inlet working pressure — undersized or restricted pipework, a failing regulator, or heavy simultaneous demand) and air in the gas line masquerading as weak supply. The boiler may keep running at reduced output, but the underlying supply question needs answering.
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 gas inlet working pressure Common
Restricted or undersized supply pipework, a failing meter regulator or exceptional simultaneous demand starves the boiler under load. The engineer measures working pressure at the boiler while it runs at maximum — the definitive test.
- Air in the gas circuit Sometimes
Air introduced by supply work or long shutdowns dilutes the gas reaching the valve; the controller reads it as weak supply. Sime's remedy includes bleeding the gas circuit.
- Partially closed or faulty gas cock Sometimes
An isolation valve not fully open throttles supply exactly like a pressure problem. Checked early because it is trivial to fix.
How to fix it
- Notice whether other gas appliances seem weak DIY safe
A hob burning lazily at the same time suggests a property-wide supply issue rather than the boiler — useful evidence for the engineer and possibly your gas supplier.
- Reset once with the OK/RESET button DIY safe
Hold the OK RESET button for more than 3 seconds. Sime allows a maximum of six consecutive resets before locking out on AL 28, so if the code returns after one or two attempts, stop and book an engineer.
- Have the working gas pressure measured Gas Safe engineer
A Gas Safe registered engineer will measure inlet working pressure with the boiler at full rate (UK natural gas should hold close to 19–20 mbar under load), check the gas cock, purge air from the circuit, and — if the supply itself is deficient — advise on pipework or refer the matter to your gas supplier/network.
- Have a Gas Safe registered engineer work through the combustion-control checks Gas Safe engineer
Sime's documented checklist for the combustion-control faults covers the ignition/detection electrode (condition, position and earthing), the flue inlet and outlet for blockage, the air diaphragm, the gas calibration and supply pressure, and bleeding air from the gas circuit where relevant. After rectification the engineer reruns the automatic self-calibrating procedure and confirms combustion with a flue-gas analysis.
Parts you may need
- Gas cock/isolation valve · from £25
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 £90–£350, depending on the underlying cause.
Frequently asked questions
What does AL 84 mean on my Sime boiler?
The boiler has derated itself because the combustion system believes the mains gas supply cannot sustain full output — suspected low gas pressure. Sime's checks are the actual gas flow/pressure and air in the gas circuit. It may keep heating at reduced power, but the supply question needs a proper measurement.
Why would my gas pressure be low?
Common reasons: long or undersized supply pipework, a failing regulator at the meter, heavy simultaneous demand (several gas appliances at once), or a partly closed isolation valve. An engineer measuring working pressure at full rate identifies it quickly; supply-side faults go to your gas network.
Is running at reduced output harmful?
Not immediately — derating is protective. But underlying low pressure can worsen and affect combustion across appliances, so treat AL 84 as a prompt for a proper gas-pressure check rather than something to live with.