A4 Boiler Fault Code: What It Means on Every Brand

The A4 fault code appears on 2 brands — Grant and Viessmann — 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: Engineer may be needed 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 A4 means	Severity	DIY?
GGrant	A4 on a Grant Aerona³ heat pump is an AC current detection error — the unit's protection against excessive current draw on the incoming mains side has operated. Full guide →	Engineer may be needed	Some checks
VViessmann	The A4 fault code on a Viessmann boiler signals that the boiler has detected a problem related to its return water temperature or heating circulation. Full guide →	Engineer may be needed	No

A4 brand by brand

Grant A4

A4 on a Grant Aerona³ heat pump is an AC current detection error — the unit's protection against excessive current draw on the incoming mains side has operated. Grant's documented remedies point at two practical checks before anything else: the installation position (a blocked air inlet or outlet makes the unit work abnormally hard, dragging current up) and the power supply itself (confirm a healthy 230V). If both are sound and the code persists after a restart, the fault moves inside — to the measurement circuit on the main PCB or a component genuinely drawing excess current. It is the mains-side sibling of A2, which covers the same supervision on the DC side of the inverter.

Causes, fixes & costs for Grant A4 → · All Grant codes

Viessmann A4

The A4 fault code on a Viessmann boiler signals that the boiler has detected a problem related to its return water temperature or heating circulation. In plain terms, the boiler is seeing return temperatures that are too high, outside their expected range, or it cannot confirm that water is circulating correctly through the system. Depending on your specific Viessmann model, A4 can point to a faulty return temperature sensor giving incorrect readings, a pump that has become airlocked and cannot push water around the system, or a build-up of sludge and debris restricting flow. On some models it can also indicate a burner-related circulation issue. Because the precise trigger varies between Viessmann models, an accurate diagnosis really does require a qualified engineer to inspect the boiler directly rather than guessing at the cause.

Causes, fixes & costs for Viessmann A4 → · All Viessmann codes

Frequently asked questions

Is the A4 fault code the same on every boiler?

No. A4 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 A4 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 A4 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.