Sime AL 80 Fault Code: Causes, Fixes & Repair Costs
What does the Sime AL 80 fault code mean?
AL 80 on a Sime boiler reports a fault on the gas valve's control logic line — including the case Sime calls out explicitly in its manual: a damaged valve cable. Where AL 77–79 concern the current the valve draws, AL 80 concerns the integrity of the control connection itself: the board has detected that the wiring or logic line between it and the SGV valve is not behaving like an intact circuit. A cut, chafed, crushed or disconnected valve cable is the textbook cause; a fault in the valve's internal electronics or the board's interface produces the same alarm. The remedy is Sime's familiar pairing: check the gas valve and board — with the cable inspected first.
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
- Damaged or disconnected valve cable Common
Sime names the damaged cable directly in the solution column. Looms can chafe against the chassis, be pinched during service work, or pull loose at the connector — breaking the control line to the valve.
- Valve-side electronics fault Sometimes
The SGV valve's own interface electronics can fail, corrupting the control line even with perfect cabling. Substitution confirms it.
- Board interface fault Sometimes
The PCB end of the control line can fail the same way; checked once cable and valve test sound.
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 boiler was recently serviced DIY safe
A valve cable pinched or displaced during recent work is a classic AL 80 trigger — worth telling both your engineer and whoever did the earlier job.
- 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 cable/loom · from £30
- 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 £120–£400, depending on the underlying cause.
Frequently asked questions
What does AL 80 mean on a Sime boiler?
The control connection between the board and the electronic gas valve has failed its checks — Sime's manual specifically cites a damaged valve cable, though the valve's or board's interface electronics can cause the same alarm. The boiler locks out until the line is proven sound.
How is AL 80 different from AL 77, 78 and 79?
Those three report the valve drawing the wrong current; AL 80 reports the control line itself compromised — often literally a damaged cable. In the workshop the checks overlap heavily, which is why Sime gives the same remedy for the whole family.
Is a cable repair cheap?
Relatively, yes — if the loom is the culprit, the part cost is small and most of the bill is the visit, at the lower end of £120–£400. Valve or board replacement pushes toward the top, with recalibration required afterwards.