High
Ext’ Interlock Off on Keston Heat 55

Your boiler has stopped operating because a safety device connected to the outside of the heating system has been triggered.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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.

What the manual says

Manufacturer manual

External Interlock Operated, Reset Interlock

From this model's manual
Verified against the Keston/Keston Heat 45 installation & service manual.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

Ext InterlockView →LockoutView →

All 36 fault codes on the Heat 55

Seeing a different code? Every code this model can display, in one place.

167475767778798081828384858687888990BCC FaultCylinder Thermistor FaultError 20False Flame LockoutFan FaultFlame LossFlow/Flue Thermistor FaultFlow/Return ReversedFlue Thermistor FaultIgnition LockoutLow Mains VoltageLow Water PressureOutside Sensor FaultOverheat LockoutPCB FaultReturn Thmr FaultToo Many Resets

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