Both use the same fault codes to indicate the issue but display them in a different way depending on what control panel you have.
If you have the manual switches the red led will flash a number of times then pause then repeat the flashes again, you need to count the number of flashes.
If you have the Duo Control Panel it will display a red “!” on the left of the display. If you press and hold the water heater button it will illuminate a number of bars, the number of bars showing correspond to the fault code number.