The A8 fault code appears on 2 brands — Grant and Worcester Bosch — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.
The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.
| Brand | What A8 means | Severity | DIY? |
|---|---|---|---|
G Grant | The A8 fault code on the Grant Aerona³ heat pump indicates that the outdoor ambient air temperature sensor (commonly referred to as the T8 sensor) is returning a reading that is out of… Full guide → | Lockout — engineer likely | No |
WB Worcester Bosch | The A8 fault code means your Worcester Bosch boiler has lost communication with its FX sensor controls or programmer. Full guide → | Lockout — engineer likely | No |
The A8 fault code on the Grant Aerona³ heat pump indicates that the outdoor ambient air temperature sensor (commonly referred to as the T8 sensor) is returning a reading that is out of range, open-circuit, or short-circuit. The main PCB cannot obtain a valid temperature signal from this sensor, which it relies on to manage compressor operation, defrost cycles, and weather compensation logic. As a safety measure, the unit will typically shut down until a valid reading is restored.
The A8 fault code means your Worcester Bosch boiler has lost communication with its FX sensor controls or programmer. In plain terms, the boiler's internal BUS (communication line) has stopped receiving the expected signals — either from a connected programmer or from a code plug — and the boiler has shut itself down as a precaution. On certain older models such as the RD532i, A8 can also point specifically to a problem with the flow temperature sensor or its wiring, sometimes causing the boiler to overheat before locking out. Either way, the boiler will not operate while A8 is active, and it should not be bypassed or forced to run.
Causes, fixes & costs for Worcester Bosch A8 → · All Worcester Bosch codes
No. A8 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.
Not usually. On every brand listed, A8 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.
For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. For heat pumps, an MCS or F-Gas qualified heat pump engineer. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.
General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.
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