Viessmann A3 Fault

Viessmann A3 Fault

This fault code A3 means that there is a problem with the boiler’s water pressure sensor or the actual pressure of water in the system. When too low or too high water pressures are detected by such sensors, this fault arises hence affecting the working of boilers and in some cases posing safety problems. Knowing what causes such faults and how to fix them is important as it ensures your heating system stays efficient and safe.
Causes of A3 Fault Codes

Low Water Pressure: The heating system may leak leading to low pressure to fall below its minimum value, or radiators that have been bled recently or loss of natural pressure over time can cause this.

High Water Pressure: An extremely high water pressure can also activate an A3 error. Such could result from an issue related to filling loop or expansion vessel.

Faulty Pressure Sensor: Even if the real pressures are within their permissible ranges, a defective sensor for measuring water pressures will produce false readings leading to an A3 fault code.

Air in the System: Air trapped in any part of the heating system will affect both water pressure readings and overall system pressure.

A3 Fault Code Troubleshooting and Fixing

Check Water Pressure: Assess whether your boiler’s gauge indicates low or high pressures. Most residential boilers usually operate at about 1.0-1.5 bars for normal conditions.

Repressurize the System: To correct low water pressure, use your boiler’s filling loop with reference to manufacturer’s instructions. Care should be taken not to over pressurize it though.

Release Excess Pressure: If you find out that it was too much, then let off some through radiator bleed valve into normal range again; be cautious not to drop too fast prior doing so.

Bleed Radiators: Bleed all radiators within your home as this might be due to air entrapment which affects readings on a gauge for instance when checking steam generator rentals etc. thus will stabilize the pressure.

Inspect for Leaks: The heating system could be losing pressure as a result of leaks which should be examined. Any leakages discovered should be fixed as soon as possible.

Check the Pressure Sensor: If there are no noticeable leaks and the pressure seems all right, this may mean that it is a problem with the sensor itself. Only professionals should replace and inspect sensors.

Consult a Professional: As A3 fault code can be caused by different factors, it is better to contact an engineer from Gas Safe Register who will provide you with appropriate diagnosis and repair if simple troubleshooting fails to address it fully.


A3 fault code on Viessmann boiler means that there is an issue connected with water pressure in the system. Proper diagnosis and resolution are essential for ensuring safe and efficient operation of boilers. Regular maintenance and immediate response to variations in system pressures go a long way in preventing these problems from occurring again.