Rheem 1l returns shortly after descaling

[Dan74]

I just descaled my tankless I used 6 gallons of vinegar 3gallsons at a time. I ran it for a total of 6 hrs, changing the vinegar out at 3hrs. I then cleared the code. The 1L code is back within 2 weeks. Could there be something else going on? We have city water and its not that hard?