The heater shows FAULT 21 error code. (Series G6)

This means the heater cannot sense the core temperature correctly.

Is this a new installation & is the installer is still with the heater?

YES

Ask the installer to switch off the heater at the supply (or both supplies) and make sure the supplies are dead. (If the installer isn't present a engineer will need to attend)

Ask them to remove the right hand & front cover so they can clearly see the Charge Controller PCB.

Ask them to check the CORE sensor cable is plugged into the PCB in the socket marked CORE1. See picture

If the sensor plug was loose or not plugged in, hopefully pushing it back in will resolve the issue. Once covers are on and heater is powered up, the heater should work OK. If FAULT 21 persists, raise an engineer visit - Take 03/82034/0 Charge Controller & 02/80010/0SP

NO

Send engineer

Take 03/82034/0 Charge Controller & 02/80010/0SP Core temperature sensor