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Last updated: 12th January, 2023


Worcester Bosch A21 Error Code - How To Fix

Worcester Bosch A21 Error Code - How To Fix

Key takeaways

  • The Worcester Bosch A21 error code indicates your boiler and wireless thermostat are no longer effectively communicating via RF signal.
  • The signal loss could be caused by: dead batteries in boiler or thermostat, a faulty component within either or the thermostat being out of effective range.
  • Always arrange a qualified, Gas Safe engineer to carry out a full boiler service.

If Your Worcester Bosch boiler is displaying an A21 error code it can be a vexing issue, especially if your home isn’t being heated as it should.

Here at iHeat we’re dedicated to assisting our customers in any way we can, whether it be stocking a premium boiler collection or giving helpful heating tips and advice. 

In this blog we’re going to be exploring the Worcester Bosch A21 error code, including: what this means, what causes this, how to fix and more.

What Is An Error Code?

A Worcester Bosch error code is simply a warning message to homeowners that their boiler isn’t functioning as it should.

Error codes (or fault codes) are alphanumeric and often 3 digits in length (A21), they are shown on a boiler’s digital display screen, usually located in the centre of the unit or hidden beneath a folding plastic cover lower on the boiler.

Due to the nature of this particular error code however, the A21 fault code may also appear on your room thermostat.

What Does The A21 Error Code Mean?

The A21 error code indicates your Worcester Bosch boiler and wireless thermostat are no longer effectively communicating, this occurs when there is a lack of Radio frequency (RF) signal.

RF is how a boiler and wireless thermostat transmit data to each other in order to effectively and safely heat up your home.

When room temperature drops below the desired level while the heating is switched on, the thermostat detects this and via RF signal alerts your boiler it needs to start emitting heat once again to bring the temperature of the room to satisfactory level..

RF Signal Loss Causes

Thermostat batteries have run out and require replacing

Find a flat blade screwdriver to carefully remove the thermostat from its wall mounting, replace faulty batteries, line up with wall mounting and re-attach, then more than likely the thermostat will need resetting to your desired temperature  settings.

Thermostat needs to be in closer proximity 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 manufacturer guide/manual, and if necessary reattach wall mounting and thermostat in new location (considering performance and safety).

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.

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.


Arranging for your boiler to be annually checked over or ‘serviced’ by a qualified engineer can be a fantastic way of analysing the state of your heating system in yearly increments, identifying and rectifying any faults before they have a chance to progress.

Be Proactive With iHeat

The best way of ensuring your boiler and thermostat are operating as they should, is by proactively replacing ageing or faulty boilers at the earliest signs of degradation, this preventative step is far more prudent than reactive maintenance.

It’s a mechanical and engineering fact that due to some of the technological advancements in recent times, modern boilers are more robust and efficient than anything made previously. 

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