How much does it cost to fix Main error code 20?

Typical repair cost for the Main 20 fault is £120-250 including labour.

20 High

Main 20 Fault Code

Q: What does the Main 20 fault code mean?

Your boiler has stopped working because it cannot light the flame, which is sometimes caused by a blockage in the waste-water pipe.

The Main 20 fault code means: your boiler has stopped working because it cannot light the flame, which is sometimes caused by a blockage in the waste-water pipe.. It appears on 2 Main models in our database.

Technical description: Flame Failure or Blocked Condensate Drain

Typical repair cost £120-250 (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 20 fault code

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

Insul 7 Repair: £150-250
System 18 HE Repair: £120-250

Source

Main 20 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