A1 Boiler Fault Code: What It Means on Every Brand

The A1 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.

Up to: Stop & check now 2 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What A1 means	Severity	DIY?
GGrant	The A1 fault code on the Grant Aerona and Aerona³ heat pump signals a discharge temperature error. Full guide →	Stop & check now	No
WBWorcester Bosch	The A1 fault code on a Worcester Bosch boiler indicates that the circulation pump is not performing as the boiler expects. Full guide →	Engineer may be needed	Some checks

A1 brand by brand

Grant A1

The A1 fault code on the Grant Aerona and Aerona³ heat pump signals a discharge temperature error. This means the control system has detected an abnormal temperature reading at the compressor discharge point — the hot-gas outlet on the compressor where refrigerant leaves at high pressure and temperature. The unit will typically stop operating to protect the compressor from damage. When this code appears, it shows on the Aerona remote controller (accompanied by a flashing red LED and a warning icon) and simultaneously as a two-digit code on the LED display of the Terminal PCB inside the outdoor unit.

Causes, fixes & costs for Grant A1 → · All Grant codes

Worcester Bosch A1

The A1 fault code on a Worcester Bosch boiler indicates that the circulation pump is not performing as the boiler expects. This could mean the pump is seized, running dry, blocked by sludge, or that an airlock is preventing water from moving around the system. Without proper circulation, the boiler shuts itself down to avoid overheating and internal damage. On many Greenstar models the display shows A1 alongside sub-code 281, which specifically points to the pump running dry or with air present. Note: on certain 8000-series Worcester Bosch units A1 can carry a different meaning — always check your model's manual if in doubt. Alongside the fault code you may notice lukewarm hot water, uneven radiator temperatures, or a complete loss of heating and hot water.

Causes, fixes & costs for Worcester Bosch A1 → · All Worcester Bosch codes

Frequently asked questions

Is the A1 fault code the same on every boiler?

No. A1 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.

Can I fix a A1 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a A1 fault?

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.