Sime AL 78 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 78 fault code mean?
AL 78 is one of Sime's SGV gas-valve supervision codes: the current measured through the valve's EV2 operator has exceeded its upper operating limit. Unlike AL 77 (absolute limits breached), AL 78 is the narrower operating window — the valve circuit is drawing more current than a correctly functioning valve should at that point in operation, which suggests a deteriorating coil, partial short, wiring leakage or a drifting driver stage. Because valve current is the board's evidence of correct gas metering, the boiler stops rather than continue. Sime's remedy is to 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
- Deteriorating valve coil drawing excess current Common
An ageing EV2 coil with partial shorts between windings draws above-spec current before failing outright — AL 78 often precedes a terminal AL 77.
- Wiring insulation leakage Sometimes
Heat-degraded insulation can leak current to earth or between conductors, nudging the measured value over the upper limit intermittently.
- Board measurement/driver drift Sometimes
The PCB can over-drive or misread a healthy valve; ruled in when a replacement valve changes nothing.
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.
- Note whether the code is intermittent DIY safe
AL 78 that appears when the boiler is hot but clears cold suggests heat-related insulation or connector breakdown — useful detail 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 78 mean on my Sime boiler?
The electronic gas valve is drawing more current than its upper operating limit allows. It is usually a deteriorating valve coil or its wiring, occasionally the board's driver stage — Sime's manual says check the gas valve and board, and the boiler locks out until that is done.
Can I keep using the boiler between lockouts?
No — treat recurring AL 78 as a stop. The fault sits in the gas-metering chain, the boiler limits your resets anyway (six consecutive, then AL 28), and an intermittent electrical fault in the valve circuit warrants proper diagnosis, not perseverance.
What will it cost?
Typically £150–£420: wiring repairs at the bottom, a replacement SGV valve mid-range, a control board at the top — plus the mandatory self-calibration and flue-gas analysis after valve or board changes. Get a written quote.