Whale water heater fault.

Hi

I have a Venus 500/4. The Whale water heater is not working, the fault code is 3 flashing red lights.

The manual says with this fault there is a low/high supple voltage. 

So I have turned all power off and on again

Then disconnected the battery and power cable, then reconnected. Still 3 flashing red lights and no hot water.

I don’t don’t know what else to try, any ideas please?

Thanks in advance.

Dan