Boiler displays E20, may indicate external control or thermostat fault

Safety first: Isolate power before working on control wiring.

• Check controls: Ensure the room thermostat and programmer are calling for heat (set to maximum temperature and constant on).
• Bypass test (engineer task): At the boiler's low-voltage control terminals, temporarily link the room stat contacts as per Sime instructions. If the boiler runs, the external control or wiring is faulty.
• Inspect wiring: Check for loose or damaged cables between the boiler and thermostat/programmer.
• Replace faulty control: Fit a compatible room thermostat or programmer if found defective.