Boiler displays E12, no response to thermostat or programmer, boiler may stay off

Safety: Isolate power before accessing the wiring centre or boiler terminal strip.

User checks:

• Ensure the room thermostat or wireless controller is turned up and calling for heat.
• Check that any external programmer is set to an ON period and not in holiday or OFF mode.

Engineer checks:

• At the Keston boiler control terminals, verify that the required link (if no external controls are used) is fitted between the correct terminals as per the installation manual.
• Test continuity of the room thermostat circuit and any wiring centre connections; repair loose or broken wires.
• Bypass the external controls temporarily at the boiler terminals to confirm the boiler responds; if it does, replace or repair the external control device.
• Check for correct voltage presence on control inputs as specified by Keston.