A7 Boiler Fault Code: What It Means on Every Brand

The A7 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 A7 means	Severity	DIY?
GGrant	A7 on a Grant Aerona³ heat pump reports a defrost temperature sensor error: the thermistor on the outdoor coil that tells the controller when frost is forming — and when a defrost cycle… Full guide →	Engineer may be needed	No
VViessmann	The A7 code appears when the boiler's control board detects that the fan has not reached a complete standstill at a point in the cycle when it should have stopped — or that the electrical… Full guide →	Lockout — engineer likely	No
WBWorcester Bosch	The A7 fault code indicates that your Worcester Bosch boiler has detected a problem with the domestic hot water (DHW) NTC thermistor — the temperature sensor responsible for monitoring the… Full guide →	Engineer may be needed	No

A7 brand by brand

Grant A7

A7 on a Grant Aerona³ heat pump reports a defrost temperature sensor error: the thermistor on the outdoor coil that tells the controller when frost is forming — and when a defrost cycle has finished — has failed or is reading implausibly. Air source heat pumps frost their outdoor coil routinely in UK winters and rely on this sensor to time defrosts correctly: too few and the coil ices into a solid block, too many and efficiency drains away. With the reading untrustworthy the unit stops rather than mismanage defrosting. Grant's documented remedy is direct — a faulty sensor should be replaced — and diagnosis is the usual resistance check.

Causes, fixes & costs for Grant A7 → · All Grant codes

Viessmann A7

The A7 code appears when the boiler's control board detects that the fan has not reached a complete standstill at a point in the cycle when it should have stopped — or that the electrical plug-in connection to the fan is unstable. As a safety measure, the boiler enters a burner fault lockout to prevent unsafe or erratic operation. The precise behaviour varies slightly by model: on the Vitodens 100-W the burner is placed into fault mode (sometimes linked to DIP switch S2 being set to ON); on the Vitodens 200-W, 222-F and Vitotronic 300/200/200-H/050/300-K and Vitocontrol-S ranges, A7 often indicates the unit has switched to a constant outdoor-temperature control mode, typically because of a short-circuit in the outdoor temperature sensor; on Vitotronic 300-K paired with Vitotronic 200, it can signal a communication fault between the two controllers. Whatever the variant, the underlying theme is the same — the control unit has detected something unexpected with the fan or a connected sensor and has acted to protect the system.

Causes, fixes & costs for Viessmann A7 → · All Viessmann codes

Worcester Bosch A7

The A7 fault code indicates that your Worcester Bosch boiler has detected a problem with the domestic hot water (DHW) NTC thermistor — the temperature sensor responsible for monitoring the temperature of your hot water supply. When this sensor sends readings outside the expected range, or stops communicating altogether, the boiler flags it as defective. Depending on the severity, the boiler may continue producing hot water but with reduced temperature accuracy, or it may lock out entirely as a safety precaution. The fault and its indicator LED will typically flash on the display to alert you to the problem.

Causes, fixes & costs for Worcester Bosch A7 → · All Worcester Bosch codes

Frequently asked questions

Is the A7 fault code the same on every boiler?

No. A7 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 A7 fault myself?

Not usually. On every brand listed, A7 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.

Who should I call about a A7 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.