How much does it cost to fix Main error code 21?

Typical repair cost for the Main 21 fault is £150-300 including labour.

21 Emergency

Main 21 Fault Code

Q: What does the Main 21 fault code mean?

Your boiler has detected that it is running too hot and has automatically shut down to prevent damage or dangerous overheating.

The Main 21 fault code means: your boiler has detected that it is running too hot and has automatically shut down to prevent damage or dangerous overheating.. It appears on 1 Main model in our database.

Technical description: Safety Thermostat Activated (Boiler or Flue)

Typical repair cost £150-300 (including labour)

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Main models that show the 21 fault code

Click a model for the full fault code list and model-specific repair notes.

System 18 HE Repair: £150-300

Source

Main 21 fault code information on this page is sourced and cross-checked against the official Main service / installation manuals for each affected model.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026