Worcester Bosch A21 Error Code

A Comprehensive Guide to Understanding and Fixing
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Published: 19 Aug 2023

Troubleshooting the A21 Error Code: Worcester Bosch


Returning home to find the Worcester Bosch A21 error code displayed on your boiler can be a real concern, especially during cold weather. This error can disrupt the comfort of your home, leaving you wondering about what went wrong. In this guide, we’ll explain what this code means, its common causes, and step-by-step solutions to help you restore your heating system.

How to Fix Worcester Bosch A21 Error Code - Trades Team

Safety First

Before attempting any fixes yourself, please ensure you follow all safety guidelines and consult your boiler’s manual. If you’re unsure about any step, it’s best to consult with a professional Gas Safe registered engineer.

What Is An Error Code?

A Worcester Bosch error code is a warning message indicating that the boiler isn’t functioning as it should. These error codes (or fault codes) are alphanumeric and usually three digits in length (A21). They are shown on the boiler’s digital display screen and alert homeowners to potential problems with their heating system.

Worcester Bosch A21 Error Code - Trades Team

What Does The A21 Error Code Mean?

In simple terms, the A21 error code means that your boiler and wireless thermostat aren’t communicating correctly. This occurs when there is a lack of Radio Frequency (RF) signal. The RF signal is how the boiler and thermostat transmit data to each other to heat your home.

RF Signal Loss Causes

Understanding the cause of signal loss can help you identify the best solution:

  1. Battery Replacement: If the thermostat batteries have run out, find a flat blade screwdriver to carefully remove the thermostat from its wall mounting, replace the faulty batteries, and reset it to your desired temperature settings. (Safety Note: Always follow the manufacturer’s instructions for battery replacement).
  2. Thermostat Positioning: The thermostat might need to be closer to the boiler. Remove the thermostat from its wall mounting as described above, move to a new location, and carry out signal tests as instructed by the manufacturer guide/manual. Ensure proper positioning following the guidelines in your manufacturer’s manual.
  3. Thermostat or Boiler’s Receiver are Faulty: Component error or faulty performance in this case may need to be assessed and remedied by a qualified Gas Safe registered engineer. Consult with a professional if you suspect faulty components.
  4. Other RF Signals Interfering with Central Heating Appliance Communication: If the source of interference is discovered, and if safe to do so, this or the thermostat should be relocated to allow for efficient communication with the central heating system. Safely relocate any device causing interference, following manufacturer guidelines.

When to Call a Professional

While some issues can be addressed with simple fixes, there are times when professional help may be needed. If you suspect a more serious problem or feel unsure about the steps you’re taking, it’s wise to call a qualified Gas Safe registered engineer.

Book an Annual Boiler Service

Ensure your boiler stays in peak condition by booking an annual boiler service with our experienced Gas Safe registered engineers.


If you’re regularly experiencing issues with your boiler, it could be time you scheduled a boiler service. Regular servicing by a qualified engineer is essential for maintaining your heating system. Look for professionals who are Gas Safe registered (like us) and ask about what their service includes.


This comprehensive guide is designed to help you navigate the Worcester Bosch A21 error code with confidence. From understanding what it means to actionable solutions, we hope this information assists you in maintaining the warmth and comfort of your home. Feel free to bookmark or share this guide with others who might find it valuable. Heating systems can be complex, but with the right knowledge, you can take control of common issues and keep your home cosy.

About the author

Trades Team are experts in Heating, Plumbing and Gas Services. We have 15 years of experience providing expert Boiler Repair and Boiler Servicing to homes and businesses across Essex, Suffolk, Kent and London. Learn more about Trades Team.

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