Sime AL 79 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 79 fault code mean?
AL 79 completes Sime's trio of SGV valve-current codes: the current measured through the gas valve's EV2 operator has fallen below its lower operating limit. The board expected the valve to be drawing a certain current for the operating state it commanded, and the circuit is drawing less — the signature of a weakening coil going open, a high-resistance connection developing in the loom or connectors, or a driver stage failing to deliver. The boiler locks out because under-current means the valve may not actually be achieving the gas flow the combustion controller believes it has commanded. Sime's remedy, as across this family: check the gas valve and board.
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
- Valve coil going open / high resistance Common
A coil beginning to fail open draws progressively less current until the lower limit trips. Often the terminal stage of a long decline.
- High-resistance connection in the valve circuit Common
A corroded spade terminal or loose connector inserts resistance, starving the coil. Classically intermittent and temperature-sensitive.
- Board driver under-delivering Sometimes
The PCB's valve-drive stage can sag with age; confirmed when current at the board output is low even into a healthy valve.
How to fix it
- 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.
- Stop if ignition attempts become erratic DIY safe
Under-current in the valve circuit can accompany failed or partial light-offs. If the boiler struggles audibly to ignite, leave it off for the engineer.
- Have a Gas Safe registered engineer test the gas valve and board Gas Safe engineer
Sime's documented remedy for this family of codes is to check the gas valve and the control board. The engineer will measure the SGV valve's coil currents and wiring, test the board's valve-drive stage, and replace whichever has failed. Because the valve is part of the combustion-control system, any replacement must be followed by the full automatic self-calibrating procedure and a flue-gas analysis.
Parts you may need
- Gas valve (SGV) · from £150
- Control PCB · from £200
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 £150–£420, depending on the underlying cause.
Frequently asked questions
What does AL 79 mean on a Sime boiler?
The gas valve's operator is drawing less current than its lower operating limit — evidence of a weakening coil, a high-resistance connection, or a board failing to drive it properly. Because that undermines the board's certainty about gas flow, the boiler locks out for an engineer's attention.
Why does the boiler watch valve current at all?
On Sime's combustion-control models the valve is electronic and modulating; measured current is the feedback proving the valve is doing what the combustion controller commands. Current outside limits — high (AL 78), low (AL 79) or absurd (AL 77) — means that proof is gone, so the boiler stops.
Is this dangerous?
The lockout is the safety response working as intended. Do not bypass or repeatedly reset it; book a Gas Safe registered engineer. If you ever smell gas, leave and call 0800 111 999.