Error U3

That's a "Low Pressure" fault and it's possible you may only need to reset the mode. This question is posted under DD603 but a U3 code is usually associated with the 602 models. I am including an instruction for you on how to check the setting. Start with that first, it is easy to do.
Let me know if you are not able to read the text.

U3 will display because the water did not drain from the previous stage in the cycle... Check the HP/LP settings first as this will give the impression the water did not drain if incorrectly set. Otherwise the pump may be damaged, blocked or the waste hose blocked.