E21 High

Johnson Starley E21 Fault Code

The Johnson Starley E21 fault code means: your boiler's electronic control board can no longer correctly read data from its internal sensors, causing it to shut down for safety.. It appears on 7 Johnson Starley models 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: ADC lockout
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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Johnson Starley models that show the E21 fault code

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

Source

Johnson Starley E21 fault code information on this page is sourced and cross-checked against the official Johnson Starley service / installation manuals for each affected model.

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