What is the difference between 诚意 and 诚信?
诚信: means honesty and credibility

诚意: means honesty only

You can't say:


  他对我有诚信。


Because if somebody 有诚信, he/she will be always 有诚信, not only for you, but for everyone. So you just say 他有诚信 or 他没诚信.

You can say


  他对我有诚意。


意 means mood here. One can be 有诚意 now, but wasn't other times, can be 有诚意 for you, but not for others.

诚信 is a trait, 诚意 is a mood.

You can say


  他今天有诚意.


not


  他今天有诚信