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E16 indicates a fault with the heater.
The service manual states; Water heating element is short circuit or defective electrical insulation.
Unplug the appliance and take the rear panel off.
Check the wiring to the heater first.
If it's ok, take the heater out and visually inspect it.
Test the heater using a megger (if you have one) to check the electrical insulation.
Repair the wiring or replace the heater to try and cure the problem.
Hope this is of some help
Let me know how it goes or if you need more help