Error A8

Safety first

1) If you smell gas, evacuate the property and call your gas emergency number immediately. Do not operate switches. 2) Before doing any inspections, isolate the boiler electrically by switching it off at the boiler switch and at the consumer unit if you need to access any wiring. Do not attempt internal electrical repairs or any work on gas components unless you are Gas Safe registered.

Initial homeowner checks (safe, straightforward)

1) Try a simple reset: use the boiler reset button or the reset procedure in the manual (often holding the reset/temperature control for 3 seconds). Wait a few minutes and see if the A8 clears and the boiler restarts. 2) Check the mains power and fuses: ensure the boiler has power, no tripped RCD or MCB, and no obvious blown fuse in any fused spur. 3) Check external controls: if you have a room thermostat, programmer, wireless receiver, or an Energy Management System (EMS) connected, ensure those devices have power, fresh batteries, and appear connected. Reboot or reset any external control modules or wireless receivers according to their instructions. 4) Note when the fault happens: intermittent after power cuts or storms suggests a transient communication problem.

Simple visible checks (power off before touching wiring)

1) With the boiler isolated from mains power, you may check any accessible external wiring terminals such as thermostat or external control terminals that are readily reachable without removing internal covers. Look for loose wires, corroded terminals, or damaged insulation. Tighten loose terminal screws if you are competent and the power is off. 2) Inspect the boiler exterior and the area around connectors for evidence of water ingress, corrosion, or burn marks which can indicate connector failure.

Diagnostics and next steps

1) If resetting and the simple checks do not clear the fault, avoid repeated resets. Repeated lockouts may mask a persistent fault and could stress components. 2) Record the exact fault code and any associated cause code shown in the boiler info menu (press the spanner/return button if available) and note whether the fault is constant or intermittent. 3) If you have external EMS or bus wiring to the boiler (for example a wired weather compensator, smart thermostat hub, or cylinder stat), try disconnecting that external device at its plug or terminal (with boiler power off) and then restore power and test. A faulty external device or bus cable can cause A8. 4) If you find a visibly damaged cable or connector you can isolate the system and leave it disconnected and call an engineer. Do not attempt to splice or permanently repair gas or boiler internal wiring unless qualified.

What a professional will do

A Gas Safe engineer or qualified heating electrician will perform safe live diagnostics including measuring signals on the communication bus, checking voltages, inspecting and testing the FX sensor and HCM/PCB connections, and replacing faulty sensors, bus leads, or control modules if required. They can also check for low mains voltage or intermittent supply issues and test the boiler under normal operating conditions.

When to call a professional

Call a Gas Safe registered engineer if the fault persists after a reset and the basic external checks, if you are uncomfortable performing the visual checks, if you find damaged wiring, or if the boiler remains in lockout and you have no heating or hot water. Also call a pro immediately if you detect signs of water ingress or burning in the boiler or if there is any gas smell. Do not attempt internal electrical or gas repairs yourself.

Summary guidance

1) Try reset and basic external checks first. 2) Do not open or work on internal boiler electrics or gas parts unless you are qualified. 3) If the A8 persists, contact a Gas Safe engineer and provide them with the A8 code, any cause/subcode shown, and details of any external controls connected to the boiler so they can diagnose and repair safely and efficiently.