A3 Boiler Fault Code: What It Means on Every Brand

The A3 fault code appears on 3 brands — Grant, Viessmann 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: Lockout — engineer likely 3 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 A3 means	Severity	DIY?
GGrant	The A3 fault code on the Grant Aerona³ heat pump indicates a Current Transformer (CT) disconnection fault. Full guide →	Lockout — engineer likely	No
VViessmann	The A3 fault code appears across several Viessmann boiler ranges but can carry two distinct meanings depending on your specific model. Full guide →	Lockout — engineer likely	Some checks
WBWorcester Bosch	The A3 code is a locking fault that appears when the boiler detects a problem with its fan or the air pressure switch that monitors it. Full guide →	Lockout — engineer likely	No

A3 brand by brand

Grant A3

The A3 fault code on the Grant Aerona³ heat pump indicates a Current Transformer (CT) disconnection fault. The CT sensor is a small monitoring device on the main PCB that continuously measures the electrical current drawn by the compressor. When the PCB loses the CT signal — either because the sensor has become disconnected, the wiring has failed, or the PCB itself is unable to process that signal — the system raises the A3 alarm as a protective measure to prevent the compressor from operating outside safe current limits. The unit will typically stop operating and the fault code will appear on both the Aerona Remote Controller display and the LED display on the unit's Terminal PCB, accompanied by a flashing red LED on the controller's on/off switch.

Causes, fixes & costs for Grant A3 → · All Grant codes

Viessmann A3

The A3 fault code appears across several Viessmann boiler ranges but can carry two distinct meanings depending on your specific model. On many Vitotronic and Vitocontrol-based models, A3 signals that the boiler has detected a high-limit temperature condition — essentially the boiler overheating and shutting down as a safety measure, sometimes linked to a blown fuse (F2) on the control board. On Vitodens 100-W and similar condensing boilers, A3 more commonly points to incorrect system water pressure — either too low or, less often, too high — which the pressure sensor has flagged as outside safe operating limits. Because the same code covers two quite different underlying problems, the first step is always to identify which applies to your boiler by checking the pressure gauge and your model's handbook. Regardless of the root cause, the boiler will typically lock out until the fault is resolved.

Causes, fixes & costs for Viessmann A3 → · All Viessmann codes

Worcester Bosch A3

The A3 code is a locking fault that appears when the boiler detects a problem with its fan or the air pressure switch that monitors it. The fan plays a critical safety role: it draws fresh air into the combustion chamber and expels harmful flue gases outside. Before allowing gas to flow to the burner, the boiler checks that the fan is spinning correctly — the air pressure switch confirms this by detecting the change in air pressure the fan creates. If that confirmation never arrives, or if the fan speed is wrong, the boiler locks out and displays A3 to prevent unsafe operation. A reset is needed to clear the code once the underlying fault has been resolved, but repeated resets without fixing the root cause will not keep the boiler running.

Causes, fixes & costs for Worcester Bosch A3 → · All Worcester Bosch codes

Frequently asked questions

Is the A3 fault code the same on every boiler?

No. A3 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 A3 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 A3 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.