Rheem Tankless- 1L Error keeps coming back

[BradASchreiber]

I have a Rheem tankless (Model RTG-95DVLN) water heater that keeps showing the 1L error code. It has been descaled twice in the last 4 months. Each time the inlet filter was also cleaned. This latest occurrence happened one week after the last descaling. I have a water softener on top of my plumbing system. The vinegar in the bucket was not that dirty after the last descaling, just mostly tinted blue. Why does that keep returning after I reset it? I know the buildup cant be that bad after a week. Thanks!

 

[Fitter30]

Check inlet and outlet temps with a k style thermometer and compare with units readings. Also flue gas. Cleaned the inlet air filter? Cleaning HX use 4 gallons of vinegar not diluted minimum 1 hour.