There is no specific error code for a faulty automatic air vent on a Vaillant boiler; however, issues related to air in the system, which an automatic air vent is meant to address, can manifest as various fault codes like 

F22 (low water pressure), F23(blocked or low-output pump), or F24 (water shortage, pump issue, or air in the appliance) on some models. You may need to check manually for air in the system or contact a qualified Gas Safe registered engineer for further diagnostics if you suspect an issue with the air vent or the system. 

Understanding air vents and their role:

• A Vaillant automatic air vent (AAV) is a safety device designed to release built-up air from the heating system using buoyancy.

• When the AAV isn't functioning correctly, air can accumulate in the system, leading to low pressure or other issues that can trigger error codes. 

Signs of air in the system:

• Gurgling or banging noises

• from the radiators or pipework. 

• Cold spots

• on radiators. 

• Low water pressure

• , indicated by a low reading on the boiler's pressure gauge, which could lead to an F22 fault. 

What to check:

• Manually check for air: You may need to check the auto air release valve and any other additional auto air vents to ensure they are open.

• Monitor boiler pressure: Check the boiler's pressure gauge. If the pressure is low, it may indicate air in the system.

• Contact a professional: If you suspect a problem with the AAV or can't resolve the issue, contact a Gas Safe registered engineer for in-depth diagnostics and repair.