Sime AL 77 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 77 fault code mean?
AL 77 belongs to the family of SGV gas-valve supervision faults on Sime's combustion-control boilers. The board continuously measures the current flowing through the electronic gas valve's EV2 operator, because that current is how it knows the valve is doing what it is told. AL 77 means the measured current has gone outside the absolute maximum/minimum limits — beyond anything a healthy valve circuit could produce. That points hard at an electrical failure: a shorted or open valve coil, damaged wiring, or a faulty driver stage on the board. Sime's remedy is to check the gas valve and board; the boiler locks out, since gas control cannot be trusted while the valve's electrical behaviour is out of bounds.
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
- Gas valve coil failed (open or short) Common
A failed EV2 operator coil draws wildly wrong current — effectively zero if open, excessive if shorted — immediately breaching the absolute limits and locking the boiler.
- Valve wiring or connector damage Sometimes
Chafed insulation or a corroded connector alters circuit resistance and hence measured current. The loom is checked before the valve is condemned.
- Board valve-driver fault Sometimes
The PCB stage that drives and measures the valve can itself fail, misreading or misdriving a healthy valve. Sime pairs valve and board in the remedy for this reason.
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.
- Do not repeatedly reset a valve-supervision fault DIY safe
This code concerns the component that meters gas. One reset to check for a transient is reasonable; repeated forcing is not.
- 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 77 mean on a Sime boiler?
The current through the electronic gas valve has gone outside absolute limits — electrical behaviour no healthy valve circuit should show. It means the valve coil, its wiring or the board's valve-driver has failed, and the boiler locks out because gas control cannot be verified. It is an engineer repair.
How is AL 77 different from AL 78 and AL 79?
All three supervise the same valve current. AL 77 is the current breaching absolute maximum/minimum limits; AL 78 and AL 79 are the narrower upper and lower operating limits respectively. In practice the diagnosis is the same — valve, wiring, board — but AL 77 suggests the starkest electrical failure.
Is the boiler safe while showing AL 77?
It has locked itself out, which is the safe state — it will not attempt to run with unverifiable gas control. Leave it off and book a Gas Safe registered engineer. As always, if you ever smell gas, leave the property and call 0800 111 999.