E66 High

Main E66 Fault Code

The Main E66 fault code means: your boiler is struggling to direct heat correctly between your radiators and your hot water taps, likely due to a stuck internal component or a sensor reading error.. It appears on 1 Main model in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description: 3-Way Valve Assy, Safety Thermostat, or Pressure Gauge
Typical repair cost £180-350 (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 E66 fault code

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

Source

Main E66 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